LOW BP? I don't know what's going on

Hello..I'm 26, female. It first started last month when after eating breakfast I used to feel that I'll fall back and my body became very heavy so I had to sit down and then my heart beat became fast and it resolved then around 20 days Back it increased and the heavy limbs feeling stayed all day. And one morning after breakfast it was so bad, I lied down and my whole body felt pins and needles. So I went to the emergency. They checked everything and said that its dehydration and have me 5 days of ors. (I didn't even understand as I was having normal water and it wasn't even that hot). Fast forward now..I'm better I don't feel heavy all day but after breakfast there will be a wave where I'll feel like the head is heavy and I'll have to sit down. I literally spent the last 15 days very sedentary. And now if I walk around and work and get stuff down for a while I'll feel the heavy wave again and I'll have to sit down. It resolves fast but why does it keep happening. I always feel like im walking around with like..5%heaviness. For context I'm 159cm, 41kgs, I've always been thin. My thyroid, ecg, sugar all came normal. I checked my cortisol is was 17.56 in am and 9.8 in pm. The vitamin d is around 20 but I'm already having medicine for it for three weeks. My iron was around 69 and hemoglobin around 11.9. (Last year it was around 9 but I had supplements to pull it up and even last year I didn't have these symptoms). Sodium is 141 and potassium is 3.7. What's going on? Is the recovery so slow? Is it dehydration again? My pressure was 90/60 at the emergency and day before yesterday it was 101/70