Need some help

Ok so Iv had 3 kids ... I was 115 lbs and i'm 5'5 before my 1st was born... After my 1st was born I went from 160lbs down to 110 lbs pretty easy with no issues ended up 110lbs at the end before I got preg with my son and I went from 110 to 160 again and then went back down to 120. Then I got preg with my next about 6 months later and now I cant seem to loose the weight at all. Iv tired dieting eating right on a 1500 diet plan .. and I just get sick I get so tired after a couple days I feel awful like im actually sick( I have 3 kids as a stay at home mom so I cant afford to feel bad) on top of that if I get hungry and wait to long to eat I get sick and I mean rle sick where I start gaging and throwing up. Iv had my thyroid checked and it was fine. Im at a loss... I had horrible morning sickness and Puppps with all 3 kids and the same thing would happen while I was preg if I got hungry and didnt eat I would start throwing up and gaging. Atm I eat alot of fruit and drink mainly water which I cant have any choc or caffine as I have heart problems which also makes exercises hard as i'm tachy and if it happens when im trying to exercise ill pass out ect. So the main thing I do is just diet eating...Also after I eat I get extremely tired .... Im not very unfit as before my 1st I ran track ect and I can still run now a good bit.. I just cant figure out why I start throwing up and get awful head aches now when I didnt after the 1st two preg. Also I try to eat at the same times everyday but sometimes this is not possible because of the kids. Obv if im late eating a meal I would expect to get a slight head ache but not throwing up specially when im not preg. Also on a 1500 diet plan I have never had a problem with it but now I just feel soooo very tired all the time when I try to diet. As long as I am eating big meals to the point im rle full I have no issues and carrying around snacks(these meals being healthy of course ). There has to be something else going on right? Any idea would be welcome.