We are going to need a lot more information to have any sort of idea what is going on.
Is it a dull ache, or is it a sharp pain? dull ache
Is there any swelling, compared to your other arm? No swelling. Both biceps are sore though
Is it limited to a local area, or run the entire length of the muscle? If local, where? It runs from the shoulders to the head of the bicep (along the tendon)
Does the pain come on during other chest exercises (flys, push up)? No, I can still do MP and also BW+20kg Dips without any pain.
Does the pain come on during bicep exercises (curls, pull ups, rows)? I can still do chinups and rows without any pain..
Does the pain come on during overhead press exercises? No
Does the pain come on when lifting any weight, or only on heavier sets? Just from 60kg and above
Maybe the bicep pain is a secondary problem along to the cuff. That could be.
I'm stretching and foam rolling it like crazy.
I do have a bad habit of flaring my arms out when I bench heavy, but seeing that I only bench once a week, I am not sure how this pain could come about from just that.