Thats a really good question.
It think in general, laughing hard is not a problem UNLESS you have complications. In your case, since laughing hard is causing a little spotting I would be extra careful - you dont want to dislodge that stitch. I would definitely ask your doctor about it at your next appointment but be careful not to get too carried away with your laughter before then because you might be using muscles etc. that might have an effect.
Gosh, I hope that makes sense. Basically, be careful not to involve your abdomen too much when laughing and ask your doctor about it.
try "head-laughing" :D it uses the same technique as "head-singing" which means you laugh through your sinuses and mouth instead of your belly...it sounds absolutely ridiculous, but the upside is that everyone else laughs with you. I had to head-laugh after my daughter was born because of the stitches for my cervical/vaginal tear, or my stitches pulled and bled. i think spade22 is right, ask your doctor, but until then just try to use your head to do the laughing and let your belly take a rest.