Hi there fionaapplelover. You've asked a few great questions here - I am going to address them in order in this reply for clarity
Firstly, that is not a weird question at all! Your bladder is in the same part of your body as the internal parts of your genitals and sex organs, so pressure on one area can lead to pressure and therefore sensations on the other. For instance when some people experience stimulation in the front wall of their vagina from a penis, or fingers or a sex toy, it also puts some pressure on the wall of the bladder, as its located next door, and so feel like they need to pee. Pressure can be felt both ways, so when your bladder is full, it might be putting pressure on and therefore simulating a sensitive part of your internal anatomy, like the internal structure of your clitoris (this includes areas you may have heard of like the "G-spot"). Whilst this likely won't be the only scenario in which you can enjoy masturbation in the future, it sounds like this pressure is giving you some extra simulation in an area where this is pleasurable for you.
You can read more about your anatomy in these articles - knowing more about how your body is set up is a great and powerful first step to understanding yourself:
Secondly, you mention feeling bothered that you aren't getting a "full" experience out of masturbation. Is that because you're having difficulty feeling aroused and wanting to masturbate in general, or feel like you're unable to derive pleasure from it? Both situations can feel really frustrating. You also mention feeling "really bad" when you masturbate, would you feel comfortable telling us a bit more about that feeling? Is it a sense of shame or that you shouldn't be doing it, or more like a sensation of pain or discomfort?
If it is pain or discomfort, then it would be a good idea to hold off trying to masturbate for a bit to work out what is causing that. If it is more a mental sense of discomfort, taking a break can also be a good idea whilst we unpack that here. Your mental state plays a really big part in feeling arousal and deriving pleasure from any sexual activity, solo or with a partner.
Finally, you mention a couple of things it could be really helpful to unpack a bit as we explore these questions with you:
1) You mention a concern about "hurting people", which you also referenced in your previous post, what kind of hurt are you worried about causing?
2) You say, "I don't even really care if I experience pleasure, as long as my partner is attracted and wanting me": a caring partner cares about your pleasure and that you are having a good, safe and enjoyable time too. Is there a reason you feel you need or want to minimize your own feelings in this way?
I've given you a lot to read and think about, so it is completely fine to ask further questions and also to answer in segments if that is easier. We are here to help and hope this is a useful starting point to explore your questions!