I'm practically sure this was part of an episode from the Spider-Man: The Animated Series (1994-1998), although I can't remember anything else from that episode.
The way I remember it, there was a tiny spider in the sink with a human head (with the face of Peter Parker). Aunt May was "fighting" it, while Peter tried to tell her something in the line of "It's me!", which she couldn't hear because he was a tiny spider. After a short sequence, she turned on the tap, and the stream of water pulled the Peter headed spider into the drain.
The next sequence depicted a giant spider walking in the city. Although it looked giant, it still had a regular sized human head (with the face of Peter Parker). People ran away, as Peter begged them for help.
What makes me think this was either a dream of some kind or an hallucination is the fact that these sequences were very short and happened very fast, although I can't remember anything about what happened before or after, so I can't tell for certain.
I tried searching up for Spider-Man animated series nightmare and dream sequences, and all I was able to find were an episode titled "Nightmare" (which doesn't seem to have that scene, as far as I could see), and a dream where a giant Venom is fighting against a giant Spider-Man costume to take over Peter Parker.