The Scotland international has struggled with a series of niggling problems this campaign, including groin and hamstring complaints.
But Forrest played the first 70 minutes of the game at Celtic Park and slotted home a second half penalty after Dundee substitute Matt Lockwood was dismissed for a foul on Gary Hooper in front of goal.
"I am feeling good. I am doing a lot of running with the sports scientist and I just need to get more games," said the 21-year-old.
"There has not been anything seriously wrong and maybe that's a good thing but it has been really frustrating, coming back and getting injured (again).
"Hopefully with a little bit of luck I can stay fit for the rest of the season."
Regular penalty taker Kris Commons was ruled out of the game with a virus but Forrest was happy to take on the responsibility.
"I was asked before the game and I felt confident," he added.
"I think that is the main thing if you are going to hit a penalty and I am just glad it went in.
"That's one out of one penalties, I have given myself a chance (of taking the next one) but I won't bother with that, it was good to get the three points."