Low Platelet count and lymes disease

My 9 year old female black lab stopped being able to use her back legs on Sunday.  She was diagnosed with Lyme's disease through a routine heartworm test 2 years ago but showed no symtoms (symptoms).  However they have had her for 3 days and her platelet count is 40,000 and she also has a low red blood cell count.  They are treating her with doxycycline.  They believe this is caused by the lyme's disease, but the question I have is could it be cancer?  She has a tumor under her neck that they don't seem to be the lease bit concerned about. Thank you in advance for your response.