I had this exact problem with Elvis get to a vet the sooner the better!
Yes its probably an infection and will need meds unfortantly theres nothing you can buy over the counter if its infected. also look in the ear not only may you see redness and maybe bleeding from scratching but also look for puss and discharge the best way the vet discribed it to me was it looks like cauliflower in there ear as you enter the ear drum.
A family members dog got an infection bad & i dont know the details but it caused some sort of ear drum rupturing & her dog had to have surgery
As far as being a normal thing no its not, but allergies yes very common in pitbulls especially certain ones elvis is blue and they are know for major skin and allergy troubles
We didnt know at 1st why he got his? now we do food allergies I ran out of food one day & just used some cheap purina type my sister had for her lab big mistake. We are now very carefull that he gets no table food etc but if he finds something it can get bad. The time i mentioned when i got him to the vet it was so far down in his ear and so painfull they thought they were gonna have to sedate him. We end it up with them doing a massive ear cleaning, gel meds, a shot to help redness and ease pain, and oral pills.
I hope this isnt it but it can get very pricey if you wait and dont get it checked out. If you have one try an spca vet they are who we use now and are amazing and so much cheaper!
I say get to a vet & have them take a look
What food do you use & does your dog shake his head this is another sign?