My cats been sitting in the litter box but not using it

Ok so roughly a couple days ago my 9 year old cat started going to the litterbox alot more than usual i normally check and she doesnt seem to actually be going to the bathroom. (and yes she is squatting as if shes trying to use the bathroom)  normally after she makes these little trips she cleans her personal area a bit excessively. ive been changing her water so its fresh and tried giving her different dry food and wet food im not sure if i should wait another day or so to see if shes just being wierd or should i contact a vet?

and my cat as far as im aware has had no medical issues, only thing that might count is sometimes she eats too fast and pukes. 

if anyone has any idea what this might be youd be a huge help  

Which does she have trouble with? Peeing or pooping? If her urine output is low or minimal or God forbid, none existent, you have an emergency on your hands. A cat can get crystals blocking urine flow and this can cause death within 24 hours (the bladder can burst). I'm not as sure of the emergency status if she is constipated. Either way, as this has been going on for a few days, I'd bring the cat in to a vet fast. Really fast. Especially if she is having trouble peeing, or if you aren't sure. Can you examine her litter to see what's going on? Any urine clumps or poo that looks bloody? Does her bladder feel full?

Has there been anything left in the litter box at all? If nothing, she needs to go to the vet ASAP.