it happens could be several reasons
1. she is self conscious of her smell down there.
2. she could be a chick that discharges a lot, gets yeast infections often
3. women get different kinds of nuts, the finger nut, the tongue/clit nut, and the deep dee nut, all 3 feel different. she just may be in the mood for a pounding nut, or want to ride her own nut.
4. you have some women who nut extremely hard from the tongue/clit nut, and its hard for them to recover. so what happens is, if you make them cum that hard to begin they out of it to go on with what ever it is you want to do following, which means she too tired to open her legs a certain way, arch her back, ride etc. so she may not want you to make her cum from the tongue so she can hang with you for awhile.
5. she thinks that by you going down on her, that makes her obligated to go down on you, and she doesnt want to sukk the dee , so she gets out of that, by telling you she dont want you to do it with her.
those all the reasons ive encountered.