Are you massaging the area good after injection? A trick I do is I take a deep tissue massage gun and massage the area real good. It doesn't take the knot away 100% but it does help and it's less noticeable.
No I don't have experience using a warmer. But I've heard it works if your gear ever crystallizes. I've always used the hot water trick with vials if it ever crystallizes.
Microwave a cup of water for 1 min 10 secs. Draw up your dose and drop the syringe in the cup. Let it sit for a min and then pin. I was having this same issue with swelling. Since doing this method it no longer happens.