I'm so conflicted. Should I (27F) be bothered that my girlfriend(27NB) only has sex with me after I've done her favors

First off, throwaway account for obvious reasons but I (27F) and my girlfriend (27NB) have been together for 5 years. In the beginning of our relationship, our sex life was strong. We did it at least 3 times a week. Although admittedly, that was a bit much, still, It was as if we couldn't get enough of each other. But for months now, I have noticed that they were getting less and less interested in sex. They were fine with makeouts but would rather us masturbate next to each other than actually having sex. Whenever I ask them what the problem is, they brush me off saying they're just tired. And whenever I make a move, they come up with excuses- sometimes even starting an argument- just so that we wouldn't have sex. We have been sexless for 2 months now. But I understand their situation and just left it alone. Now, I have been doing some side hustle apart from my main job. I do paitings and and pottery. And just last week, my partner came to me for a commission. I usually do them favors so it was nothing for me but they suggested that as compensation for the low pay,, they could have sex with me. To be honest, I was kinda giddy at first cause we haven't been intimate in weeks but now that I think about it, we might've gone longer if they didn't need that favor. It feels like they are using sex as a leverage because they know I won't say no. And to be honest I kinda feel insecure because of this. It feels like they would only have sex with me if I did something for them in return. That having sex with me needs to be compensated with some favor I did. I am so conflicted and I can't ask my circle because they are our mutual friends.

Has anyone been in this situation before? Please help me. I really don't know what to do...