Neck lump/ weight loss

Last summer I found a sizeable lump under my jaw, and waited more than a month to have it checked as recommended. The GP was concerned about the lump - but I felt fine besides a sore throat that has lasted. My CBC came back clear. However, I was told to wait six weeks and if it was still there to get booked for an ultrasound. I've still not had that scan due to circumstances but will be having it next month. Between last summer and Dec I lost over 5kg, and I'm now at the lower healthy BMI range, edging towards underweight. I also get tired easily now. My GP took another blood test - which results I'm currently waiting for. 

I understand that this new blood test is precautionary due to the weightloss I hadn't experienced when I intially saw my GP. I was wondering how quickly can something change in relation to my blood? Or can an issue truly develop that quickly?

M