Other than the advice already given , I would say "take it easy".. If you get to a point where you get headaches, just stop.
Often your meditation is related to your expectations of it.
For example you may think "I'm going to meditate for 30 min", where you may get dizzyness after 10-15 mins..
I would say that quality meditation is better than quantity.
10min of quality meditation is better than 30 mins af lazy , dozy meditation.
Dont have expectations. Just focus on 'quality awareness'.
If you lose your focus just quit. And take an "Aspro Clear"