If you have taken corticosteroids lately, they can cause problems with the Achilles tendon. Something about taking coricosteroids in conjunction with antibiotics can actually cause this tendon to "snap" or break. (I can no longer take corticosteroids because of the side effects they caused me. Including awakening with tendons so hard and painful I thought they would break if I stepped out of bed too hard.)
I haven't heard of those types of problems, maybe research the medication and look up potential side effects. But more likely your over used, or pull your muscle and it just needs time to repair. To help reduce imflammation perhaps have some anti inflamtory pain killers eg. nurofen/ ibrufren (same thing), maybe have some proteins and rub the area with creams like deep heat or metsal which helps muscle tenderous. and REST the leg.
If it keeps getting worse maybe you should see a doctor...