What Causes Unexplained Bruises On Body?

im 40 years of age.. 5'8.. i recently had a thyroidectomy in jan..2011 im taking a generic brand of synthroid 100mg.. and im having unexplained bruises popping up on my arms.. and through out my body.. so im wondering is it coming from the medication im taking.. or the sugery i recently had.. because ive never bruised like that before until i had the thyroidectomy in jan..2011