When to Test with Irregular Period?

I have a somewhat irregular period due to still nursing my toddler, and was wondering if anyone has any advice on when to take a pregnancy test. I just had 3 days of light spotting and cramps - was thinking it was my period starting (which has been missing since late Feb, btw!), but nothing more ever happened. I took a pregnancy test a few days ago and it was negative. I really don't want to test again until I've waited long enough to see if this spotting was anything. Anyone know how long I should wait?