Random nausia, stomach aches (naval area)

I've been having some trouble with my stomach lately, it's not the first time I have, but this time is different. Two years ago I had this...thing (can't remember what the doctor diagnosed it as), where as soon as I'd eaten a meal or snack my stomach would bloat, I'd become gassy and generally be in agony for hours afterward. He gave me tablets to take before a meal that did something to the lining of my stomach.

Now, however, I've been getting random episodes of nausia and some instense stomach aches. They started around a month or so ago, sometimes in the morning when I woke up and I'd feel groggy, other times throughout the day. It starts with a stomach ache that intensifies until I start to feel really sick, I'll go to the bathroom just in case I vomit, I get the feeling in my throat, but I never actually vomit. I normally just empty my bowels, which brings on the stomach ache even more but eases slightly afterward. After I feel like the nausia is passing, I'll get very cold and shaky and my arms with get goosebumps. I've only ever heaved once, but I'd just woken up, so nothing was able to come up as I had an empty stomach.

I'm 17, 5ft 2", having safe sex, very irregular periods, generally healthy and a good weight. My biggest fear is pregnancy and/or some lifelong disease/condition, I'm rather afraid of being sick, a stomach condition would be one of my worst nightmares :/

Could the two incidents I mentioned (two years and now) be related in any way?