Severe Side Effects on amitriptyline, which other med should be tried?

IBS-D, Finally had a GI consult yesterday and he wanted me to try Amitriptyline. I usually tolerate medication well and had high hopes.

Unfortunately I woke up this morning (after taking it last night around 7pm) and came within seconds of feinting while I was in the shower. This happened again while I was making breakfast, both times I had to immediately sit on the floor and take deep breaths. Also ended up being constipated all day.

Wondering if anyone has experienced anything similar with this drug and if Nortriptyline or another TCA was more mild and better tolerated for them.