Waves of nausea

Run Laura Run

Hello
I am a 36 year old woman and have been running for many years with around 75 races under my belt, mostly half marathons. I am now training for my 2nd marathon (last one was 2008) and have naturally upped my mileage recently and adapted my usual training for marathon training. However on a few occasiond recently I have experienced sudden waves of nausea to the point i think i may throw up. I had one last week during a steady 6 mile run with club mates (8min mile pace) and then again yesterday in a particularly tough half marathon about 10 miles in. Both times the nausea subsides and doesn't stop me but i am worried about the marathon and the possibility of it getting worse. I haven't changed my fluid or food intake and have had the same routine for all of my training. These were not exactly long runs. I usually train and race at between 7.20 and 8 min miling and am planning on 8 for the marathon.
Any advice would be appreciated.
Thank you