Panic
Am a recurrent sufferer: have you had it badly before?
If you've had it badly before and more than once, suggest you get ABs immediately. Leaving too long can lead to scarring and even more recurrence as I know to my cost.
In meantime, drink half a pint of water every half hour, add bicarb of soda or one of those proprietary cystitis products...
This might head it off but if it starts to hurt more, you feel feverish, you get cloudy/bitty urine or pink/bloodstained urine get yourself to an out of hours clinic straight away for the ABs.
Someone else (thanks!) suggested the rather undignified nappy solution if you have any in the house and are likely to leak on the way...
Some surgeries will accept your story over the phone and send someone else in with a clean jar containing a sample - and will send ABs that way.
A bath is probably only necessary if you need to wee in it (I know, gross, but the logic is, if you have cystitis you need to empty your bladder fully each time, if you're in pain, the bladder goes into spasm and can't empty properly. Sitting in the bath or shower to pee can actually help...) If you don't want to, don't need to do that, a hot pack or hot water bottle can help.
Take painkillers also for same reason above to do with emptying bladder - ibuprofen is the best thing to take but you can alternative with paracetamol or even cocodamol if you have any.
Don't drink alcohol, have sex or eat/drink anything too acidic - may make it worse...
Good luck - is horrible and I know being a MANY times cystitis veteran