Thank you for your reply Mrs Cunningham, it's very appreciated.
I took her to the vets to be on the safe side. They told me that dogs do sometimes go off their food when on heat but they wanted to check her over more thoroughly. They dipped her urine which showed traces of blood, protein and a high PH. Her vulva didn't appear swollen so although the vet said she couldn't be 100% sure whether she was on heat but she thought it more likely to be a UTI. She's prescribed a course of antibiotics. She said the high PH may be suggestive of crystals so when we go back in another couple of months for her to be spayed they're going to test her urine again to see if the PH is still high. In the mean time I am going to try and increase her water intake to flush everything through. Obviously she already has fresh water always available to her but I'm going to add a little warm water to her food too. I considered giving her a wet food instead of her kibble but they don't appear to be very high quality so decided against it.
Yesterday she ate boiled chicken for her dinner without issue. This morning she ate her usual dry food with a little warm water added too. I don't know whether the antibiotics can help that quickly but I'm just happy she's eating again!