T.S.
Is she used to you playing with her/entertaining her all the time? If yes, then it's going to be a big adjustment. At 20 months old she is still very much a baby and is probably more confused than jealous.
I think reading and snuggling with her while you are nursing is great. But I would also start encouraging her independent side. Set up toys and activities that will hold her interest. At that age they love stacking (blocks) and filling and pouring. They also like crawling in and out of things. Get some big empty cardboard boxes and cut a few windows in each, she will love "hiding" from you.
Like I said, if she's used to your undivided attention it will be an adjustment. But learning how to play on her own is a GOOD thing, she'll be an all around more content and happy little girl :)