I am having a bit of a tough time on e few levels. I already used all my stars to upgrade my weapons.

If I replay past levels will I get more stars to upgrade? or you only get them the first time around? 

Yes. You really start racking up stars when replaying levels several times to get the best rating.

If you're playing on Normal, considering bumping down to easy. After you get a feel for the stages on easy mode you'll have less difficulty when you go up to Normal and Hard. You'll also get more Stars along the way. I also recommend blowing up and shooting every object you can on one play through, find as many hidden items as you can and on your next play through you'll know where there at. Plus any files you collect, stay collected and if you get a lot that's always a good way to bump up your grade. Also there's a single First Aid Spray in every level, so always been on the look out for it.

Plus you get more stars by going to different difficulties.