I get something like this too - like Taz says, just before a period. It got worse in my last 4 years on the pill and is a bigger problem now.
It often comes with thrush or thrush-like symptoms or 'nearly' thrush. Constipation does not help, but does not cause it. I just had a really big cycle, with v sore breasts etc and a big drop in hormones which resulted in 8 pairs of soaked knickers and a skirt (I wasn't wearing tights or trousers for obvious reasons). I have previously had a total accident just before a concert I was singing in but had my concert dress to change into, and another time soaked the car seat.
Things I am trying include cutting out anything that irritates the bladder (coffee, alcohol), drinking plenty water, upping the vagifem, using multi-gyn actigel to avoid thrush, avoiding constipation. I try and go to the loo as soon as I notice I need to go.
You can be prescribed things to calm your bladder, like oxybutenol, but as it is a very intermittent problem and they can make constipation worse, I am not keen for myself.
I would get your urine tested, just in case.