Sounds to me that the potent shit was spiked, and the weak shit was fake.

Best bet now dude is to keep trying different sources until you find good quality, unaltered weed. You can adjust the 'strength' by smoking less or more - but at least you know the weed is good. If you get a good batch from 1 person, keep buying there, establish trust and most important to you - make sure the guy is consistent with quality. Smoking dope is great but only if you establish yourself first. Most stoners have at least 2 reliable dealers - advanced smokers grow their own.

And for christ's sake don't buy from black people.