I had similar issues a year ago. Worse at night...not much during the day. Finally went for an MRI and it was a slightly torn meniscus. No idea how it happened but needed surgery. Sorry to say if that's what it is (get it checked out) but you'll
be out of commission for awhile between recovery and rehab. My surgery was 7/25 and I was able to start physical activity by Labor Day. Slowly.