You certainly can overdose on Vitamin D3, but just go get your levels checked by the doctor. It is a simple blood test easily done.
How much vitamin D3 are you taking? Recommended daily dosage is between 400 and 2400 IU, depending on who you talk to, although tests have been done with subjects taking up to 10,000 IU with no reported overdosing problems. Also, you should be taking it with a meal containing fat, since D3 is fat solluble.
Maybe there's something else. I'll presume you've discontinued your vitamin D3 supplementation?