I was just walking over to my cactus farm and all of a sudden a vignette appears blurring my vision and i hear a beating heart really loudly. This goes on for like 4 seconds before it dissapears. What is this?
That's odd, but the beating heard and the blurred vision is definitely a warp effect; You might have gotten some warp from an enemy.
But if you're not into Thaumcraft yet, there is nothing to worry. Warp is basically the "penalty" of doing "forbidden" research; aka Dark-Magic.
Usually that's a sign of an eldritch guardian. And in this modpack, until you have at least enchanted thaumium armour (or ideally thaumium fortress armour because of the huge defense boost) it's best to avoid eldritch guardians. If you were near a floating purple obelisk, that is probably what spawned the guardian. You really should stay as far away from those things as you can, until your armour can withstand being outside during a blood moon and you wouldn't suffer too much from even a wither effect (eldritch guardians are pretty awful in GTNH).