so uhh, the other day i broke a string, and i wasn't really thinking, and i took off the rest of my strings and then headed out to get some new ones. I play in drop C, but just for the hell of it I decided to get some smaller than i usually get gauged strings just to see how they feel and sound and everything.
I got back and put them on and tuned it up and on the first fret on one of my strings it sounded ****ed. the note wouldn't sustain at all, and just sounded ****ty. I thought maybe I ****ed something up, but maybe just going back to a heavy gauge would fix things, so I did, and everything was working out ok, but then I noticed as i got to the higher frets of the higher strings things started going out of tune. I checked the tuning 5 or 6 times, checked harmonic tuning, everything was good. I've become known as "the tuning nazi" in our band, i know what im doing, but somethings off. like for example the 19th fret on the highest string should be an octave of the string below it, but it sounds horrible. if i go down to the 7th fret and pretty much anything below 14th, it sounds alright. idk whats wrong, im not really that good at hardware. maybe the neck is warped, or the truss rod needs messing with or something? idk. if i could, id like to fix this myself. im pretty broke. i could use some help, any advice?
Your intonation is off. Just take it to a shop for a set up. Don't go twisting on that truss rod by yourself. It's such a simple thing, but you could pay dearly for just a single solitary dumb-fit.
that happened to me once but i was able to fix it myself, but i guess it also matter what kind of bass you own and how well you know it/how to maintain it
I am going to combine the best of the three previous posts.

Bumper is right, if you haven't done this before, taking it to a shop is a good idea. One overturn could be damaging to your bass. I messed up a guitar doing this once. But while your are there at the shop, ask the technician / luthier to show you how its done right, so you can make future adjustments yourself.
Who's talking about the truss rod? It's his intonation that's futzed up and that only requires use of screws on the bridge. As long as you detune the string before changing anything and use the right size screwdriver it's impossible to do anything irreparable. If he wants to go back to small string gauge, then ignore this, you'll need a truss adjustment.