Hello janet! I've had this problem in two of my four races! The first was a total suprise, and fairly minor, the second was worse, I always run in black cyccle shorts,
and sweat so much that I don't think anyone noticed!! I'd read some advice here to the effect of drinking plenty up until 1 hour before a race, then no more until the start.
I think it is ok then to drink a reasonable amount literally as you start, because something seems to switch the circuits to "conserve fluid" the moment you start to run.
Some say energy drinks are better as they hold the fluid in the body, i felt I experienced exactly the opposite, but this possibly because I only drink the energy drinks when about to race. When I'm drinking masses anyway.
the second time this happened it was pretty alarming, and I had to dive into a gateway, I was really worried about doing my first 1/2m at Bristol, fearing correctly that there wouldn't be many secluded gateways and I couldn't rely on being at the back for privacy!!
Armed with all the advice, I made sure I drank plenty the day before, and then a reasonable amount the morning of the race up to an hour before, nothing then until the first drinks station, and I was absolutely fine. (My intended last-wee-but-one turned out to be the last as the queue at the loos was so long and slow!)
My 2nd race in May was race for life, and as the gun went, I shot off in the oposite direction to everyone else to the loo, I was fine then too.
On training runs, the last thing I ever do is nip to the loo before starting, If I can start a run with an empty bladder, - all is fine, if I start with even a small amount, I'm in trouble. I really can't run and hang on!! I have no bladder problems otherwise at all.
I'm sorry this is so long-winded, but I really sympathise and hope this helps, it's an embarrassing problem, but hearing forumites discussing their bodily functions sso openly is pretty liberating!!, if yu have trawled around these threads you will count yourself lucky our problem is relatively odour and solid free!!
I hope there is nothing wrong when you get your tests done,best of luck, Dee.