@jane567,
I was in a similar situation many years ago. For me, it turned out that my achilles tendon was almost torn completely...only a few strands of connection left.
See a doctor and find out what your situation is and don't guess or put it off.
After the MDs feel the tendon, they will x-ray or do an MRI. Only then will they know for sure what the status is. If it's torn ..even partially, the longer you walk on it, the worse it will get. If torn, you'll need to be off that foot for 4-6 weeks to allow it to heal properly (if lucky) or have it surgically repaired if torn as bad as mine was.
Climbing stairs was murder when it's torn..you just can not push off..and hurts like hell.