Newb question. Transmission problem

Hello to you all. Im new to the forum and the world of dodge. I bought my ram 1500 a couple of months ago and recently it started to make a thud type of sound when downshifting from over drive. And when it goes in over drive sounds like it slips. If i drive it on the highway for a while and then get off and drive around the city for a bit it doesnt make any sound or thud when down shifting. Can it be low tranny fluid? I dont know how to make sure is right.