It sounds like you didn't get the bearing races and lock nuts tightened correctly. It can be a pain to get it right without the correct tools. You want to get your bearing races at a sweet spot where there is no wobble if you shake your tire side to side, and no spots it catches while spinning, or any undue friction. It can be a pain in the ass at first, but when you've been dealing with a coaster brake (that you occasionally use), you should get used to re-adjusting your rear hub. The brake pads and it's assembly inside the hub can get wrecked and lock up too. I advise to only use the coaster brake if you absolutely need to stop as fast as possible, and maybe only if the rim brakes fail.