My 21 mth old's thumb is red & swollen & painful to her.?

She had a cut near her nail from days ago, I thought someone wouldve treated it since I was working, 2-3 days later its swollen (other side, opposite of nail) and its very painful to her. Ive cleaned the other side with peroxide and bandaged it like she wanted, but Im still worried. What can it be and what should I do.