Cat suddenly breathing heavy

My almost 3 year old cat has been her usual cheeky self, slept with me all night as usual, been playing and running around with our other cat this morning.  Then as quick as a flick of a light switch she's having trouble breathing.

She sounds like she needs to cough but no amount of back patting is making her.  She's really ruttling and struggling for her breath.  I am so scared of what's happened so suddenly as I say an hour ago she was running around playing.  There is nothing in her mouth and she's not eaten anything as they hadn't been fed yet.

I've tried calling my vet to ask for advice but can't get through, just ringing. 

I'm panicking as I don't know what to do or what's happened suddenly

Do you have any emergency vets in your area?  I know they are expensive but your kitty really needs to be seen by a vet.

I am with the PDSA which help people on a low income so can't afford to pay for another vet.

Touch wood she's asleep next to me now and seems to be breathing much better now asleep, she also at some lunch.  Not a clue what it is/was but I am keeping a very very close eye on her.

Sorry for posting in panic, just she's my baby and I adore her and couldn't get an answer from the vet so was a little scared.

Thank you