Could be a partial rupture in the Achilles tendon, or lower tendons of the calf.
A few weeks ago, I was struggling with a loaded shopping trolley on a slope, and felt something go "pop" in my left calf, but halfway up. I assumed I'd torn a muscle and went home - couldn't walk properly on it, and being my left calf, it was painful to drive. DH had, the previous week, torn his hamstring, and I'd googled to see the best treatment - RICE was the advice.
So I blithely googled my own injury, expecting to see the same advice - and was quite shocked to see large lettered advice to seek medical attention as soon as possible, because of the possibility of compartment syndrome if the muscle tear bled too much.
So I went to the GP, who gave me a referral for ultrasound, which does deal with soft tissue damage. Luckily for me, I didn't have compartment syndrome (he did chuckle a little at me but agreed I was right to see him) and the ultrasound, a few days later, showed the tear and that it was healing.
Yours, otoh, sounds like it's not healing properly at all, if it's swollen that much.
So - advice = go to the GP or minor injuries unit (NOT A&E) and get a referral for ultrasound so they can see what you've torn/ruptured and fix you up.