I have a knot on my foot that has been there for almost 3 months it's been getting bigger and today my foot started to swell what could it be?

Foot swelling: It is not clear what you mean by a "knot" on your foot. It sounds like you should see a doctor for an evaluation.

MRI and Biopsy: When it comes to any mass which increased in size over a short period of time, I suggest having an MRI evaluation and possibly getting a biopsy.

Evaluation, I-incyst: It would help to know just where this "knot" is located. Is it hard, soft or boggy? Is there any discoloration? Any pain or numbness distal to the knot? Depending on the answers, this could be something as simple as a ganglion or synovial cyst. To remove all doubt, and obtain a definitive answer, Have it examined by a foot specialist.

I suggest you: go and find out...you don't state where on the foot it is...so I am not sure if you are describing a ganglion or something else, but you should have it checked out and get a definitive diagnosis.

Varies: There are a number of potential causes, from inflammatory, arthritic, traumatic or other causes. I would not hesitate to be seen for an accurate diagnosis.