Yup, that sounds exactly like what I experienced. I felt that for the first time maybe 2-3 months ago, in many years of playing tennis . I was switching to a 2H backhand that I hadn't hit in 12yrs & was kept contacting the ball low in the stringbed where the sweetspot is on very old frames (an old habit I guess) but the Extreme 2.0 Pro I was demoing was very, very harsh there. Was also mishitting that 2hbh every few shots which didn't help. Anyways it is definitely from harsh shocks. It was in my left/non dominant hand but recently I felt it in my right hand from poly that gradually stiffened up while playing. You really should take at least a week off to let it heal or you can do some damage there. I took a 2 week break (got sick anyways) after playing on it for 3 days (counting the first time I felt the slightest hint of discomfort). Most of the pain went away in the first 2-3 days but some pain lingered for 1.5 weeks.
Are you using the 18x20 or 16x19? If the new 18x20 is anything like the previous one, then the sweet spot isn't huge. I think you just need to give your hand some time to heal and adjust your timing & getting used to finding the sweetspot on your new frame.
You can stand to lower the tension a bit too. I don't find multis to be the rocket launchers that most ppl think it will be at lower tenions. I played NRG2 @ 52lbs on a 16x20, 98sq inch in college & did not find control issues. If you have the 18x20 you can go even lower.