I was doing some heart meditation following a YouTube video (I was trying to study but feeling too stressed so I thought some meditation could help), but I actually had all these bad feelings. It felt like my chest was too heavy and I felt an urge to open my eyes.
Do you have any idea why?
I'm not an expert of yoga or meditation at all, actually I've started being interested in it only recently...
Not all meditation practices are equally effective for everyone. Apparently you found one that did not suit what you needed at the time. You mention that you were feeling extremely stressed when you started. You probably could have used a technique to help you relax your body and your breathing before you started.
You don’t mention anything about what this heart meditation was, but I suggest you try another type of meditation. You can find a very simple meditation technique that is extremely effective for bringing peace of mind here: http://www.ananda.org/meditation/learn/technique.html. See if you find it helpful.
On that same page, you can also choose to watch instruction in meditation by Swami Kriyananda or sign up for an online course.
Blessings on your journey,