How often are you watering? What sort of exposure does the plant have?

I've been watering every other day on these super hot days and about twice a week on the cooler days. Tomorrow will be cooler I hope it will perk up

Also it's in full sun

Gently pry apart the growth to find maybe an insect that has made a web.

Remove it.

Something damaged the tips. They will develop side shoots
below the tip and those will bloom. In fact, if you were to prune the
existing shoots back about halfway, you would get a bushier plant with
more blooms.

It's a plant you can kill with too much pampering.

"Anyone know if I'm under watering,"

Only you can answer that question. Buy a moisture meter and stab it in the dirt around the plant ... is it wet or dry?

I doubt you are underwatering - you would see a general limp over-all wilting, not tip die-back. I also doubt that a 90F day is too hot for Russian sage. My neighbors have theirs in the hell strip, unwatered, in NM where it's been over 90 for weeks. And they are thriving and multiplying.

It's possible that you are overwatering, but that would make the whole plant droopy and sickly. Use the moisture meter to control your watering, not how YOU feel or what the calendar tells you.