Keep a close eye on her.
She could be having an allergic reaction to something. You can try a small dose (about 2mg/kg) of Benadryl and see if that works. How much does she weigh (approx)?

Sariss, many thanks for your response. This morning her swelling had subsided somewhat but brought her to the vet just in case. He said it's likely she was stung and reacted. He gave her a cortisone injection and a general wellbeing check. It's now 12:30pm and she is back to normal. I asked the vet if I could have given her a clarytin or something similar and he said yes but only a half tablet. It's good to know for the future. She weighs 6kg today!! Hope she doesn't get much heavier!!