Hey seed. It's all part of the learning process bro, no worries. :smokebuds:
When you've been flushing the cups, how much water have you been running through each cup? You'll be better off using fresh, pH'd water each time you flush instead of reusing your runoff.
Also, if you're comfortable doing it, take something like the tip of a wooden skewer or similar and very carefully pull some of the soil back away from the stem. Does the section of the stem that's below the surface of the soil look skinnier than the part above the soil? Maybe it's just the camera angle, but it almost looks in the second and third pic that you might have a little bit of damping off happening. I had some seedlings that I watered too much and they looked pretty similar to yours.