Whenever I do endurance training (for example running on a threadmill) everything goes well and all, but after a while (normally 25-30 minutes) I suddenly get nauseaous and feel like I could throw up any minute. Needless to say I have to stop running and go home instead resting as it doesn't go away for one hour or so. The strange thing is that is has nothing to do with me being tired, I feel like I could go on forever, but this annoying reaction my body gets is interupting my workout session, leaving me frustrated.
Now for the potential reasons/causes, I don't know if most of this has anything to do with it, but some of these reasons may be the cause:
I have in the last 6 months been losing alot of weight, not for health reasons (somewhat, I'll come back to it) but rather for having the wrong diet. I never eat breakfast or lunch (due to lazyness), this is something I'm working on to improve, but eating breakfast have never been something I've made a habit of (except from when I was a kid of course and my mother made breakfast for me). I struggle to believe that is the major reason though, because I've been fine earlier, and I usually eat something before I go work out so I have some energy to consume.
One of the reasons why I've lost some weight is because I've been practising and "preparing" for eventually joining a football team again in one of the norwegian divisions, plus that I'm joining the army later this year, so I have to be in good shape. I was never in "really bad shape", but I decided to lose some body mass for the purpuse of focusing solely on endurance and agility instead of building muscle mass and strength. I am now very light (45 pounds or 20KG below the average norwegian, but I'm also shorter than average so bear that in mind), and I'm wondering if this sudden weight loss may be one of the reasons behind this. Perhaps my body was used to having more weight to carry etc, and now it's still not completely used to maintain the same type of workout causing me to get nauseaous. If this happens to be the reason I'm expecting this to go over by itself, and eventually it will adapt and I'll be fine again. Maybe I'm simply pushing my body to hard.

I would love to get some input from you guys if you know anything about this or have any ideas what's wrong. I would hate to join the army/football team/whatever having this problem around me, and I would love to see this go away!
Thank you for reading.
