Mazal tov on the birth of your baby.
Whether or not your are now in niddah depends on a few factors.
1) It's not clear from your question if you saw that you were bleeding prior to finding the pink on toilet paper. How soon after relations did you see that you were bleeding and how/where did you see it?
2) When you saw the pink on toilet paper, how soon was it after urination? To see why we are asking this, and to understand the various issues related to seeing stains on toilet paper, please read our article "Toilet Paper".
3) What color are your pantiliners? What size were the stains you found on them?
4) Did you feel any pain related to intercourse? Regardless of the answers to the above, if you can establish through a physical exam that there is a non-uterine source of bleeding or irritation, such as a lesion, or irritation to your stitches, etc, this bleeding would be considered blood from a wound, dam makkah. Dam makkah does not render a woman niddah.
Please get back to us or your local rav with more information. If you suspect another cause of bleeding, you may wish to schedule an exam with your doctor or a niddah nurse (bodeket taharah) trained to look for dam makkah. Until your status is clear, you should separate from your husband out of caution in case you are niddah. If you conduct yourself as though you are niddah until your status becomes clear, the day on which you began to conduct yourself accordingly could count as the first of your five days.