Does this message after the Undyne fight mean that I am no longer on the genocide route?

If your save point says "You feel dread in your body but you tell yourself to STAY DETERMINED", then yes, you probably aborted the genocide route and are on the path to a neutral route. -Did you miss any monsters in any areas? -Did you get 'But no one came' in all areas?

after undyne, my save point said "you feel dread in your body but you tell yourself to STAY DETERMINED" instead of "determination" or "## left"

In the Neutral Route, the only message which mentions a feeling of dread appears at the SAVE point named "Waterfall - Hallway" (right after you see Undyne for the first time). I will therefore assume that this was before you got very far into Waterfall.

please help me figure out what happened.

According to the Undertale Wiki, you need to do all of the following to do a Genocide run:

• Exhaust the random monster kill counter in every region. This requirement must be done before approaching the final encounter of that area.
  • [Snip extended content about how the kill counter works]
• Kill every boss and miniboss (Secret bosses So Sorry and Mad Mew Mew are not accessible, and Glyde is optional). All mini-bosses (excluding Snowdrake and Lesser Dog and including Shyren) can be encountered even after the kill counter is exhausted, which can be used to obtain extra EXP.
  • Toriel
  • Snowdrake
  • Doggo
  • Lesser Dog (random but mandatory encounter)
  • Dogi
  • Greater Dog
  • Papyrus
  • [snip post-Snowdin content]

If any of the listed bosses or minibosses are still alive, you failed the Genocide run and need to start again. You will also fail the run if you don't exhaust the kill counter before you encounter the final boss of each region.

Note particularly that both Snowdrake and Lesser Dog are mixed in with "regular" random encounters. They are not strictly guaranteed to appear, although it is rather unlikely that you would make it all the way through the rest of the kill counter without seeing them at all. If this happened, you could repeatedly spare enemies until you encountered the missing miniboss, as sparing enemies does not abort the Genocide run. In most cases, this is not necessary, but if it does become necessary, you must obviously begin sparing before the kill counter hits zero. Slightly less obviously, if you have encountered neither of these minibosses, then you must begin sparing when the counter reaches two, because killing either miniboss will decrement the counter.