Welcome to healthcaremagic for your query.
With decreasing platelet count
report being negative, most likely reason seems to be a viral/bacterial infection.
You've not mentioned about total blood counts etc.
I would advise :
1. Take bed rest
2. Take plenty of liquids orally e..g water/juices etc.
3. Medicines like paracetamol/ibuprofen
for fever/pain etc
4. If your doctor felt there is possibility of bacterial infection - he may suggest some antibiotics.
Wishing you speedy recovery
Dr Tejas Shah