Lower back radiating round

POdds - feel the fear do it anyway

I have been suffering for months (well over a year) with one-sided lower back pain which over a period of time gets worse as it:
- radiates round the top of the pelvis till the whole of the right side aches
- painful to turn over or sit up in bed (but not to lift leg)
- goes from being dull ache most of time to painful all the time

this culminates in a major spasm during which the pain is such I can't lie down, sit or stand but end up hobbling around waiting for the drugs (OTC and over max dose) to kick in... (last time was the night of 24th / 25th Dec - merry christmas all but there was an interesting documentary on glaciation on at 3am!!)

After a week during which the spasm subsides and the pain reduces I enter into the "barely any discomfort" phase of a few weeks, sometimes months before it all builds again

I am a 47 y-o female, overweight (yes I know!) but also completed an ironman just over 6 months ago and would be grateful for any potential ideas as to what the problem could be before I give in and finally see my GP.......(I like to be able to head him off at the pass!)

PS - it is not a sciatic down the leg distribution of pain but having had the odd bout of that the intensity of pain during the spasm is similar or worse

thanks for any ideas floated