Suggest Treatment For Soft Lump On The Back Of Head Of A Child

Hello, my 6 and a half month old daughter was playing around the other day rolling over when she rolled over abit to hard and hit the back of her head on our wooden floor, we didn t see any cause for concern as we didn t think she hit her head to hard and she was acting completely normal after, then the next day we discovered a big lump on the back of our daughters head, we were told not to worry by family and friends as unfortunately babies will bang there heads now and then and as she has no other symptoms there is no cause for concern. But now 2 days later we have noticed the lump on the back of her head a little smaller but is now soft and feels watery to touch, she is still acting her normal self but the soft lump concerns me, is this normal?