I think there several possible causes of this pain, depending on how long ago the surgery and what you were doing when the pain started.
After any knee surgery, it is very important to pay strict attention to the exercise limitations that your doctor
gives you. One of the most common sources of pain after a MCL repair surgery would be 'over doing it' with respect to returning to physical activity. This could result in reinjury of the repaired tendon or delayed healing of the surgical repair. I would definitely get in to see the surgeon
who performed the surgery as soon as possible. In the meantime stay away from physical exercise until you have been cleared by the surgeon.
Bursitis is another common concern after any sort of knee surgery. Fortunately, bursitis is less serious than a reinjury to the repaired ligament, and it tends to get better with anti inflammatory medications and some rest. Again, the orthopedic surgeon
will be able to tell the difference between the two problems and give you advice.
In the immediate period after a knee surgery, it is also important to think about joint infection. If you have redness or warmth of the knee or fever, those are more serious signs and need immediate evaluation. But again, please see your surgeon as soon as possible.