<human>: Can you succinctly summarize the text in 1-3 sentences?

Me and my current girlfriend have been seeing each other for 9 months currently and to date she has given me oral sex once. Afterwards she rushed to the washroom and brushed her teeth. 

It was very enjoyable and I would like to experience that again, but I'm under the impression that she may not enjoy the act of giving oral sex, due to her immediate rush to the bathroom the last time that it occurred.

I've performed cunnilingus on her so much that I cannot possibly count and helped her achieve climax I'd say around 95% of the time, through this method alone. Although I do not want it to be a tit for tat thing were there is a score because as I've read that it is unhealthy for a relationship. 

I do think however that it would be nice to be receive it once in a while, if she is willing. 

Specifically in the context of a scenario such as the following one: Engage in early morning cunnilingus with my girlfriend until she achieves climax. She then notifies me that she is too sore to engage in coitus due to vaginal soreness from our previous nights session. Which I whole heartedly understand. In this context I would like to be a recipient of oral sex afterwards (again if she is willing).

I however cannot see how I can phrase this is a manner that isn't seeming tit for tat as in "I performed on you, I would enjoy some reciprocation".
<bot>: How does one ask for oral sex, phrased in a polite manner that isn't a tit for tat phrasing?