Since you seem to have problems pointing the finger to what doesn't work on your own (and if you try and can't), I'd seek the advice of a reliable sexologue if you can, at least if this bugs you enough and you seek to keep a working relationship with man, or eventually you'll run into problems (not to mention that typically with loss or lack of desire if it persists or has always been there you might well end up aggravating the problem if you start worrying about this and don't address it). There may not even be a problem as such (and it may not be on your side either but not having met anyone you like having sex with), but just to be able to find out what's up and how to change this if it happens to be something you want to change. It's often as simple as getting to understand yourself much better and what you like and what you don't (or what prevents you from totally immersing yourself in a sexual relation), and learn to communicate this better with your partner(s).

It could be that you're trying to meet their expectations and going through the expected motions, when it's a different mood, situation, time, attitude, practices etc. that would motivate/excite you. There must be turn-offs involved somewhere (it could also be a physical problem, from hormonal to chemical deficiencies, alimentation or simple lack of sleep and such and a check-up isn't a bad idea - I'm guessing if you were on meds that could alter your libido your doctor would have told you), but it's far more typical for lack of libido to be related to psychological elements - anxiety, blocks etc.), you just can't pinpoint them atm.

That said, a lot if not most people go through temporary phases of lower libido.