I'm very worried

Last night I noticed that Cassie had a lump inside her ear which had blood on it. I'd noticed the day before that she seemed to have one of her eyes shut and wondered if she'd been in a fight. I decided to take her to the vets today to check it out. On examination the vet wasn't concerned about the ear or eye but did notice some ulcers round the mouth which I hadn't spotted. He opened up her mouth to find big ulcers inside near the back. Now that doesn't sound like much - it certainly didn't look like much to me - but the vet is very worried, as he repeatedly said. Aparently the two most likely causes are an immune system problem where her antibodies start attacking here or cancer. He kept on saying "I'm very worried". So now I have to take her back on Monday for a biopsy (which she's *not* going to be happy about) and then we'll see what the prognosis is. I want to cry. I love Cassie very much and she's a happy little cat living in Union St (going by the purring). She seems fine but the vets voice keeps coming back to me - "I'm very worried". If it's treatable she's insured so it should be OK. But I'm worried.