Any number of things could be causing it. Problem is you need an exam. someone needs to look at you and determine a cause. However, in general if your this far out and it is getting better you are more likely to not have rhabdo but you need to be certain. If you choose not to see one (I highly recommend you do) icing and rest with an NSAID such as motrin or naprosyn is the orthopedic recommendation usually assuming you have no intolerances to that type of medication. Main thing is to take it easy, rest, hydrate till your urine is running clear to very light yellow. Eat well and monitor the things you were told to look out for.
I would not recommend a jump into an exercise program (at least for the first 2 weeks) doing jumping pullups and negatives. It certainly is a very good way to improve pullups but you need to remember that if you haven't done this activity before or it's been a long time, you need to condition your tendons, muscles, and ligaments to do it. Unfortunately, for most folks, that means slow restricted movements making sure your doing the exercise properly for a couple weeks. You will see a greater payoff in the long run by learning to do it correctly versus starting out wrong and getting injured later. My personal opinion is body rows are the best way to start out if you can't do a pullup and your just a beginner. Give it a try and see.