Yeah, I found that most poses in yoga can get quite intense when you hold them for extended periods of time.
For example, when I was doing yoga regularly, I was also studying YIn Yoga, and I went into a pose, and I went as deep as I would for normal yoga, and 2 minutes into to the pose, I had to pull back out of it because it was so intense.
You do really need to ease into poses if you plan on holding them longer than a minute.