masturbation?

It sounds like you're just not comfortable with doing this. It's understandable that sometimes you might have a rush of your hormones which makes you do this, but afterwards you feel bad about it. It could be related to something that has happened to you, which makes you view it as something bad. Don't do anything that you're not comfortable with. You'll stop feeling gross when you're comfortable with it.

You stop feeling gross, by stop feeling guilty for doing it.

I dont remember feeling guilty for it at all.. but then, how do i stop feeling guilty?

so i should stop doing it then? Thanks for the advice

If you're not comfortable with it you can either stop or give yourself time to slowly adapt and get used to the idea of doing that.

well its not like i do it because i think i should do it. I mostly just feel like doing it and then i do it. Hm.. well i dont really know why i feel bad afterwards.. maybe.. when i got my first boyfriend i had never touched myself. But he was.. he really wanted it. For some reason i never said no. Didnt dare. So the first time i actually touched myself was after i had already had sex multiple times with him

If you don't want to do something, you should say so. In a relationship you don't have to do everything the other person asks you to do. This could be part of the reason why you feel bad after you do it.

I know, i know.. dont know what i was thinking. I just loved him and wanted to make him happy. Yeah.. maybe. so should i just stop doing it? For how long?

Give yourself some time to get comfortable with the idea. I can't put a time on it for you. You have to go by your own feeling. Take small steps.

but im not uncomfortable with it! If i was, i wouldnt even do it. its just afterwards i feel gross and stuff

Maybe there is some emotional issue you're not even aware of, and that's what makes your tummy feel weird. Or that may not be it, but it sometimes happens to me that I feel tired or have digestive issues if I've been ignoring my feelings.

After masturbation I find I sometimes feel dirty and other times roll over contentedly and go to sleep, depending on how I see the experience as it's happening: do I see it as a "waste of time" or "cheap endorphins"? Or do I see it has an experience of connecting with my body (and my boyfriend if he's there too) and experiencing something beautiful?

You may find this blog post by Girly Jona interesting: http://girlyjona.tumblr.com/post/15999584321/a-jerk-off-description-in-the-second-person.