In April I have done 40.85k and in May 21.8k. This is a lot less than previous months.
As you can see my plan of swimming during lunch time wasn't really working, the problem was serious headache. The headache is felt at the frontal part of my head and it was so severe that I had to stop before reaching the other end of the pool. This is something that I haven't experienced before ever since I picked up running last year so I was quite scared. I experienced throbbing in my head before, possibly due to the heat when it was hot, when I run but never felt the pain I experienced in the pool. Last week the headache sustained for a few days after the swim.
I searched for an answer.
This is called exertional headache which is quite common (once for one in a hundred people). One of the ways to avoid that is to monitor the heart rate to tell that intensity is getting too high. I can't do that in the pool and I don't feel the heat while swimming so I can't tell if I was working too hard. Breathing is also said to be a possible cause and I have to admit that I have overestimated my ability and certainly breathing with my strokes wasn't quite the same as in running.
Another article here in MedicineNet could be scary if you are near my age.
I'm not giving up so I tested my body again this morning by walking/running for nearly 4k. I have a cup of water and banana before the run and brought with me a bottle of water making sure I'm hydrated. I ran only 5~7 minutes and listened carefully to the build up of headache.
Relieved, the headache was very mild and it was gone as soon as I cooled myself down.