My check engine light is on and i had it tested, the computer says "Transmission

If it is leaking a bit, it is losing fluid. If you have a burning smell under the hood, it is probably dripping onto the exhaust and/or cat. converter. It is slipping because it is low on fluid and cannot get the proper pressure to shift clean. The lower gears will show this issue more because you use them more often. Fill it up with fluid, see how it reacts and get the leak fixed.