The more babies suckle, the more milk you produce from the stimulation. It's quite normal for new babies (and old ones too for that matter) to want to suck. There's nothing wrong with baby sucking your finger/thumb or his/her finger/thumb. Some babies are suckier than others too. Carry on as usual and speak to your midwife as face-to-face talking through problems is always better. I used Kamillosan when I had sore nipples but everyone finds different things work for them.
i used the same for my crcked and sore nipples