I'm sorry no one replied before ><
I made a post the other day and no one replied lol, if it's an issue and no one knows the answer then unfortunately you might not get a reply.
It is always best to get anything unusual checked out by a vet. We can give our opinions but none of us will know for sure, there are no qualified vets on here from what I'm aware of. It could be a spot, an infected quill/hair molecule, an abscess or worst case a tumour. If it gets bigger you need to go to a vet immediately. I just went to the vets yesterday about my boys swollen lip, turns out it was just inflammation but he said if its something that gets bigger then its more of a serious issue like an abscess or tumour.
Please post a photo so someone more experienced can try and help you but I do recommend you take her to the vets to be on the safe side. Its always a good idea to get them checked over once a year so if you haven't been yet you could go for a health check and mention the pimple then. Keep us posted