What Causes Brownish Patch On Neck/knee In A Child?

My six month old boy had low platelets count in October last year. It was lower than 10 and they gave him 3 shots of some stuff called Globum. Anyway, his platelets are back up to 256 and since the hospital he had some brownish patches appear on the back of his neck, front neck under chin and one under the back of his knee which was slightly bigger.