Hello I pulled a muscle in the back of my leg on Monday but I have woke up this morning and my ankle has swelled up?

Best to Check.. I don't know just how many days ago Monday was. It would be helpful to know if the muscle you "pulled" was in the lower leg or the upper leg. The lower leg might possibly involve a calf muscle injury or an Achilles tendon problem. The swelling might be the body's reaction to a tear or instability in the tendon. To remove all doubt, it would be wise to have it evaluated for other possibilities.
Need Ultrasound. Ankle swelling starting 5 days after a calf muscle pull can be from a venous blood clot, which is readily diagnosed by ultrasound. This is important to diagnose because vein clot can go to the lung (pulmonary embolus). More commonly, ankle swelling may be the result of too tight an ace wrap around calf or sleeping with the leg lower than your heart (like in a chair).