Sounds like Cystitis-over the counter remedies available from the chemist such as Cystopurine and Cystemme. Could possibly be Urinary Tract infection though which is best to see G.P for as can worsen and lead to kidney infection. Think best to consult G.P if you've not experienced anything like this before-best to get professional diagnosis and if you get it again in the future you'll recognise the symptoms and then be able to self-medicate. Hope you soon feel better.