I had the same problem, got incredibly sore for a week. Soreness is nice up to a certain point but this kind was just crippling.
The problem is that your muscles swell up and squeeze the nerves, cause they are surrounded by a pretty tight fascia and there's only so much room. This causes pain.
What helped me is training them DC style. Every rep is done with a 5 second negative and a 15 second pause in the stretched position. You do 10-12 reps. (Feel free to correct me if I'm wrong)
This way you train your muscles well, but you also stretch the fascia a lot. You'll get sore, but not in the same bad way. Try it.