What Causes Drastic Weight Loss Despite A Normal Thyroid Test?

I have a 19 yo daughter who is losing weight. She only weighs 110 and has now lost 5 lbs. We had this same episode last year. She had pain in her L femur and hip. Her lymph-nodes in her neck, under her clavicle, groin were randomly enlarged. Her physician ordered xrays for the long bone pain, but they were all clear. I am concerned. She had had mono approx. 8 months before these symptoms last year. We finally chalked it up to possible residual from the mono. However, her lymph nodes in her armpits were enlarged again a couple of days ago. No deep bone pain, but now starting with weight loss. We had her thryoid numbers checked and bloodwork. Nothing hugely remarkable. My question to anyone is this: Are there times when lymphoma can hide itself until if finally pronounces itself? Do any other cancers present this way such as osteosarcoma, rhabdomyosarcoma or hodgekins? Were xrays enough? Thank you