I have an ulcer in the inside of my lip which is extremely painful and i've had it for 3 days and it is getting bigger and more painful.

It could be: It is likely an apthous ulcer which should go away in 7-10 days. If it persists see a dentist for further evaluation. An over the counter topical anesthetic such as zlactin-b or orabase can help with the pain.

Can be very painful: Mouth ulcers are usually small often appear on the cheeks...Inside the lips..And on the tongue that can take upto a week or two and sometimes can get bigger which can take longer can leave scar..Should see dentist if worsening..And stays more than couple weeks.. Take extra care of your teeth and gums..Try to reduce stress..And eat healthy balanced diet..These are some ways t help prevent in future.