1.I have a fret buzz on my bass E string, and its becus i changed my strings to lighter gauge. if i change back to original size, will it not buzz?
2. Sometimes, i even have a buzz(not fret) on my 2nd and 3rd strings while playing clean.. i think its coming from somewhere near the pickup.. do you know what it is? i just adjust my pickup height becus if was too high.
1) Probably the buzz is caused because the string is sitting too low in the nut slot. You either have to go back to heavier strings, or replace the nut with the properly filed slots. If your looking for a quick fix (pre-gig on short notice) you can stick a piece of paper under the string in the nut untill you can fix it for good.

2) sometimes buzzes are REALY hard to pinpoint. They can sound like they are coming from one area and realy be somewhere else. Try strumming open strings while holding different pieces of your guitar. I generaly start with the tuning pegs (all of them, not just of the strings that seem to buzz), string trees, etc. etc. All the way down to the bridge, then try your volume and tone knobs and the strap buttons (I've had the strap buttons rattes and it was REALY hard to find and a real pain).

I guess since it started after a pickup adjustment it could be the spring in your pickup mount is too small for the low setting, and is rattleing bevause there is no tension to keep it in place. If this is the case you can just replace the spring, or raise your pickup again. If your pickup won't raise, the mounting screw may have come out of the plate, in which case you need to remove the pickup and put the screw and spring back in place. Hope this helps.

Hope ths helps!