I'm gonna throw out some random answers based on my experience as a male in the field though I'm sure females can go through this as well from time to time. People may agree with me or disagree, but these are just my thoughts on the matter.
You're a human being. You're a sexual creature. Many things can affect that sexuality. As a fairly new MT, you're going to be experiencing situations you normally wouldn't experience on a regular basis. You'll be putting your hands on other people and making them feel good but you'll be doing it for a living and with multiple people. You'll hear moans, you'll see pleasure on their faces, and you'll have more skin to skin contact (meaning your hands, arms, or maybe feet if you're ashiatsu) than possibly the clients significant others. This can have some different effects:
1) Increased physical activity. Not saying you're a lazy slob but, if you're doing your body mechanics correctly, giving a good massage can be like doing Tai Chi. Increased physical activity can do wonders for the libido.
2) Confidence. The reactions of your clients might give you a huge boost of confidence if you're a people pleasure by nature. Doing what you feel you were born to do can give you that "Me Tarzan! *beats chest*" feeling (LOL) if you haven't been feeling like that lately and that feeling can go straight to your fun stuff.
3) Connection and Old Taboos. I'm in the camp that we're a touch deprived society. I'm not saying everyone should throw Cuddle Parties or anything, but your lifestyle can be affected by the culture your society accepts. And it's gotten worse as I've gotten older in regards to touch.
When I was in high school chorus, the first thing we did as a warm-up was to turn to the right and rub the neck and shoulders of the person in front of us for a couple of minutes. Try asking someone how many schools can get away with that now. LOL Depending on what area you live in, Public Displays of Affection may or may not be looked down upon so there may not have been as much physical connection in your life as you are now getting used to with your new career. And, since our society is touch deprived and we're conditioned to believe too much touch = sex, this could be a natural programmed response in your libido based on how you've been raised. How do I know this theory of mine might have some weight? I now live in a country where there's legal prostitution, people go topless at lakes & beaches, and fully nude at some thermal spas even with small children sitting right next to them (that even had me cringing and feeling icky and I'm still getting used to it), you see nudity in TV commercials about freaking CHEESE and on billboards...yet the sexuality of the general populace here (based on observations of ex-pats) can be downright boring compared to the more "conservative" countries like the US, Britain, etc.
What I'm saying is that your body could be responding to something it's just not used to yet might of been craving even in a non-sexual way, but may be affected by social programming. I can't get a decent burger in this country to save my life and I KNOW when I visit the DC area again, I may just eat myself sick at the nearest Five Guys burger joint. I know they're bad for me when consumed every day, multiple times a day...but what if I was raised that ground beef was the spawn of Satan's Bowels or some nonsense? I'd have this whole guilt-complex that would disturb me yet thrill me since I'm rebelling against a social norm that I really don't believe in deep down.
Anyways, there are my possible answers for you. There are probably more but this post has become a Wall of Text as it is. Whatever you may be feeling at the moment, the way I've stayed legit is to keep this stuff simple. Don't agonize over it. Acknowledge it and move on. Client has a nice (insert body part or characteristic here)? Fine. Acknowledge it to yourself (NOT THE CLIENT), move on, and get back to work. Client's moaning sounds like a 1-900# operator? Acknowledge it to yourself, laugh inwardly, move on, back to work. As far as I'm concerned, whatever you do after work is your business as long as it isn't endangering someone and is with anyone OTHER than one of your clients unless you've already broken the professional relationship and waited the minimum amount of time stated by the rules and regs of yadda, yadda, yadda. Stay professional and all this stuff will just become one of the barely-there trials and tribs of the business.
Good luck!

*Just once, I'd like to type a post without a half-dozen typos...it's a dream of mine...*