Yes, I have had the same thing happen and as you say the pain is always on the opposite side that you are laying on.  Mine is worse if legs are curled up rather than straight out.  I've had RA for 15 years and I'm on Humira and Methotrexate and both have helped me quite a bit and this leg pain is a constant problem.  Also have feet that always hurt.  Just read an article about PAD (peripheral artery disease) which causes leg muscle pain caused by blocked arteries in the legs. Can be tested with a ankle-brachial index (ABI) test which just takes a few minutes and is non-invasive. I'm going to ask my Doctor about doing this test as I really think this has nothing to do with my RA unless it's from the medications. Good luck to you!