Some commenters here talking about anxiety, but I think the bigger cause, which many people don't know, is that THC significantly increases your heart rate despite it's usual characterization as a depressant. If I recall correctly (big "if" considering the circumstances hah) my heart rate after smoking would go up by 10-20 BPM (from 65-70 to 80-90) while still feeling relaxed; ~~finding some numbers on this from a reputable source is difficult right now and this symptom is suspiciously missing from the wikipedia page~~.
Edit:
Realized this comment sounds like fear mongering, so decided to dig up some actual sources. The wiki page I needed to find was:
Edit:
Realized this comment sounds like fear mongering, so decided to dig up some actual sources. The wiki page I needed to find was:
https://en.wikipedia.org/wiki/Effects_of_cannabis
Also, the CDC page mentions it:
https://www.cdc.gov/cannabis/health-effects/heart-health.htm...
And links to this paper (though I can't read past the abstract bc no institutional access):
https://doi.org/10.1002/j.1552-4604.2002.tb06005.x