Oh this could be so so so many things.
You could have bad breath. You may be a bad kisser. You may not have turned her on. You may be annoyingly affectionate. You may not be affectionate enough.
It could be you. Think about it.
OR..... She is seriously worried about her reputation. She's trying to fix it and she slipped up. She figures you are a good enough friend that you wont be spreading the rumors around that it happened. Suck it up and be a friend. Or stop being her friend and tell everyone and let it totally backfire. Then sleep with a different skank and get an sti. Then get your friend drunk, sleep with her again and give the sti to her. Then blame her for it saying that you have only slept with her in the last year and she will be mortified entirely. Then tell her you still accept her and would like to take her out on a date. Since she will have no friends and no prospects she will likely be totally down.