I’m in the 5th week of flower and one of my plants has red lines on the main stems and also red pistol hairs in the buds.

That temp may be causing some heat stress. Have you grown this strain before?? 80 seems high for flowering . .Just my .02

What have your temps been - lights on and lights off? Also, what has the temp been of the water that you give to the plant? Typically that happens when temps are low, like below 68F, or water temp below room temp.

you using any UV light? thatll cause it also

How does the branch feel? Strong, soft? How are the branches of the leaves, strong weak? Out from under the LED, how green are the leaves, light or dark?

Besides that, you said it in your statement you "recently added cal/mag" and this happened. Cal/Mag is 20% Nitrogen, or so right? And if you're feeding her grow and bloom nutes did she get a full feeding of nitrogen as well? Perhaps an early sign of nitrogen toxicity.

Have you grown these genetics before??

Could be genetics, . . . Could be stress . . .

I have the same on my photo sativa. If you know you're keeping up on your basics, see what happens (within reason). These are the experiences that make you into a gardener/grower. Just make sure you note how it turns out .