My 8 month old baby fell back into the concrete and its been 30 minutes now and she has swelling on the back of her head. should i take her to the doctors?

Most falls from their sitting height have minimal momentum and impact injury. The swelling would be from a broken blood vessel under the scalp, but these swell until the pressure stops the bleeding & the swelling usually fades over a week or so. If the kid acts weird enough to spook you, take it in. If the kid cried & stopped & resumed normal play and activities you can monitor at home.