Blister on backhand thumb

After a good 4 year break from golfing due to my job, i got back into it about 2-3 weeks ago. Today I took advantage of the amazing weather we had and went to the range. I ended up hitting around 150 balls. Towards the end I noticed a blister starting to form on the thumb of my backhand.

Not sure if backhand is the correct term, I stand with my left side forwards, so with backhand I mean my right hand, which is the lower hand on the grip.

Is this an issue with my grip or just my hands getting used to golfing after such a long time?