Melinda was Lactating way before this scene was even added and back then she knew she wasn't knocked up. This was due to the fact that later on when she fucks Daniel and he blows his load inside her she was praying he would knock her up with a boy.
My point being is that her lactating during that scene wouldn't make for a good logical excuse for her to think she is pregnant but then again Melinda despite having a massively sinister nature isn't the brightest crayon in the bunch. I mean she actually Believed Ann when she told her that Daniel was gay despite all the sexual shit they were doing afterwards.
I think she wanted to believe he was gay because it was a feather in her cap by turning him back to women. Melinda is controlling and a very manipulative person. But the truth is she wanted have done anything with daneil if it wasnt for the doctor and his condition. Sure he would have gotten a few feels here and there but if you recall she always stopped him before.
When she had to finally take it to that level because the doc told her it could kill him things change. Add in the fact she wanted a boy because seeing daniel i guess dredged up all those motherly feelings in her.
The whole lactating thing in the beginning was true, she just had amazingly big boobs but I think its different now. Melinda never said anything about it being safe for daniel to cum in her because she is on the pill. Thats why i think she is pregnant.
Think Ann told daniel she is on the pill, dont think Rhonda mentioned being on it either.