Lampard, who spent 13 years at the London club before leaving this summer, scored late on to cancel out Andre Schurrle's strike.
He told Sky Sports News: "It's a really difficult one. I'd be unprofessional if I didn't come on and do my job. I tried to get back in the box, it was a great ball from Milly (James Milner).
"I had 13 amazing years with Chelsea fans. So I'm mixed with it. Obviously, I'm really pleased the team I play for got a draw, so it a really hard one for me to answer.
"I'm a bit lost for words. I didn't expect to come on and score like that. I come on and the Chelsea fans are singing and that's emotional, then I'm playing for this club who have taken me in brilliantly as well. So I'm really stuck in the middle here. I suppose this can be seen as a happy medium in the end."
Lampard is Chelsea's all-time record scorer with 147 goals.