Hi Varsettie. Looking at the pics and vid, it definitely does look like he's undergoing a rather dramatic color change.
He seems active and his fins aren't clamped, both indicators that he's not sick. I think as long as he continues to act healthy, we can chalk this up to marbling. Someone should name these fish "chameleon fish" because they sure do like to change colors on us.
Ah. The erythromycin is also a gram positive antibiotic so again, just like the tetracycline, it wouldn't have any effect on the vast majority of bacterial infections our bettas get.