Here are a couple of websites about parasitism in monarchs, maybe you can match what you see with one of those shown:

http://monarchparasites.uga.edu/parasite...

http://www.monarchlab.org/Lab/Research/t...

I have tons of Milkweed plants. On this one plant, I have like 10 caterpillars. They all look normal with 2 antennas on both sides. But this one on my plant has 2 more in the middle, making 6. Also, at the base of each antenna, there is a red dot. Wtf is wrong with this caterpillar?