Hey! I am female, and I would personally be very flattered if a male were brave enough to tell me directly that he liked me, even if I didn't reciprocate the feelings. Who doesn't want to be liked? I think it was very sweet that you did that (unless you are really creepy, but I am assuming you are not).

The girl may be nervous if

1) she has never looked at you in that way
2) she HAS looked at you in that way, but never intended to act on it, and sees now that it is within the realm of possibility
3) she may be nervous because her boyfriend is the jealous type

Who knows? Just try to act natural, interact with other girls, and if you really only want to put her at ease, perhaps you can casually mention that you have started dating someone else (not too quickly or she will know you are lying). The awkwardness will fade away with a little time.

Sorry it didn't work out for you; you sound very sweet.