Hi. I'm Japanese college student living in Japan.
Well, "daisukidesu" and "aishiteru" are pretty similar expressions and sometimes they mean the same feeling but we can say the first one for suggesting almost everything (not only for people but inanimate things as well) and it sounds much colloquial. It's like... you know "I love you(it) bunches!" or something.
"aishiteru", this phrase is not that frequently used I suppose, I mean Japanese people take it kinda too serious to mention. So if you're now saying some lady said to you "aishiteru", it definitely meant she loved you seriously. You know not as a good buddy or anything.
If you do too, you can answer like "watashi (boku) mo aishiteimasu."=I love you too.
(I gotta say that "anata mo aishiteiru" posted above by another guy is wrong...."anata mo aishiteimasu." means "You love me, too.",which is inappropriate for this situation.)
If you don't like she does, I think you might as well show your sincerity, saying "Sorry." or something. :)
I'll leave my e-mail addy so that you can ask me when you get another prob in Japanese language :)