Small Lump on Dog?

I have a 3 year old Chihuahua and I discovered that he has a small lump on his side. I took him to the vet already and they prescribed medication saying that it might be an allergic reaction to something such as a bug bite. They did no blood tests or anything so they can't be sure. After giving my dog the medication as they prescribed it for 10 days, I have now noticed had the bump is still there, but now there seems to be a scab and some hairless around the area.

What do you think this could be? Should I take him back to the vet or could this be a sign of healing?

By Mercedes from Chicago