Why do muscle get harder?

Sounds like a spam question i guess, but it isnt.
 
Years ago when i trained i noticed my muscles (lets say biceps for sake of argument) getting harder in no time at all, when i stopped they went soft and even if i tensed them they didnt feel particularly hard.
 
I started up again mid last year and within no time they were harder again.
However, if i train my biceps one day, then later that same day they will feel softer and will feel harder in a day or so later.
 
I know as you train blood is pumped into your muscles, and the muscle sinew gets more dense as it repairs and grows, but why does it get harder? Is it due to the sinew, blood, oxygen or fat deposits getting smaller?