Get to a doctor NOW.
I had the same thing two years ago. It turned out to be a DVT or deep vein thrombosis. These clots can break off and lodge in your heart lungs or brain. This can be fatal.They do a doppler test to find it. Better that it is in the lower leg than in the thigh.
My clot is gone now, but, I will be on blood thinners for the rest of my life. I go to the Coumadin Clinic every few weeks. They do a finger-stick and based on how thick or thin my blood is, then, they change my Coumadin dose or not.
If it is not a clot, better safe than sorry.
Ulcerative Colitis since about 1976
Pan colitis since at least 1994
Never had a flare until MD gave massive dose of Magnesiun while hospitalized for bronchitis.
During subsequent hospitalization for the flare, GI looked me straight in the eye and said "don't ever stop using Asacol."
Insurance company thought otherwise so I'm on no meds.