Blood on his sheet

My son is almost 5 months old and he hasn't been himself for a few days. He's been crying a lot and might have a sore throat. This morning I noticed some watery blood on his sheet (picture below) and wondered if this could be his gums as he's not quite teething yet. He has sharp nails so I wonder if he could scratch his gums in his sleep as he sucks his fingers. What do you think?