i've just removed a few tubes from my chassis to straighten it out, but as i removed the tubes it started changing sizes in other places.
Before i just clamp it to the right size and reweld it, I was wondering if the chassis is meant to have that kind of stress in it or is it just my shit welding?

The heat from welding can result in all sorts of stresses being added to your chassis.
To keep the stresses and distortion to a minimum, I'd suggest the following.

1. Ensure all the items are completely mated where they are to be welded
   
2. Don't pull the items to be welded together with clamps; they should be an easy fit.
   
3. Keep the welder properly adjusted; just enough amperage to carry out the weld.
   
4. Tack the entire structure together first before fully welding.
   
5. Carry out welds in stages, then there wont be too much heat build up.
   

I cut a few tubes out of a fully welded chassis, so as long as some stress is expected it should be alright.

cheers