It's perfectly normal. I used to be a veterinary assistant at a cat clinic. Happens all the time. Some cats never do it, some out grow it and some do it forever. I had a cat that was part florida bob cat and extremely aggressive but when no one else was around he'd lay with me and suckle clothes, skin, whatever he could suckle on. Got him when he was 3 days ond and he did it until he died at 9 years old. As long as she's not biting and it doesn't bother you let her. Once those kitten nails get sharp you'll probably want to stop her.