I don't think it is a pimple but rather a canker sore. These happen. https://www.webmd.com/oral-health/guide/canker-sores#1 Different things can cause them such as stress or eating something like acidic like citrus and various other things. They normally do go away. This is inside your mouth but not outside? If you smoke a lot of weed, maybe not a great idea. But that's up to you. May aggravate it.
You can treat pimple with over-the-counter (OTC) medications such as salicylic acid, sulphur and benzoyl peroxide, which have proven to be very effective and safe.
Should meet to an ENT expert.