@chingky, were you tired or out of breath when your headache started? It could be na adjustment stage lang. It's possible that it's been a long time that you didn't do any exercise (just assuming) then suddenly you pushed yourself a bit too much. Baka you can avoid it by proper pacing lang.
@Hestia,
1. running, hiking, brisk walking, swimming and other cardio activities/exercise can help built stamina and endurance. That's one way, another way is make use of your gears. Kung kailangan mag-granny, use granny, huwag pilitin baka ma-blackout ka pa during uphills.
2. I often switch sitting positions, like sit normally, then sit on one butt-cheek for a few minutes (sometimes alternating between left and right). Stand up for a few seconds. These helps blood circulations and minimizes pain in the you know where. During downhills on off-trails, I'm basically off-saddle, this is both for safety (high saddle set-up kasi ako) and avoids bouncing on the saddle (especially if you have a hardtail). You can also experiment on saddle shapes and padding thickness or materials, some are more comfortable than others and that usually is a personal preference. There are also "saddle cushions" you can buy. These are like padded gloves for the saddle, they cost anywhere from 350-1500 pesos. And lastly, like feline13 mentioned, sanayan lang.
3. Lakambini is a female specific outdoor brand. You can find these in R.O.X., Habagat (SM Megamall), Conquer (Robinson's Pioneer), Bombproof (Robinson's Galleria), and other outdoor shops in Trinoma, Farmer's, and Market-Market. But if I'm not mistaken, this brand is more attuned to hiking gears. Not sure if they already have padded ones for cycling. Another one would be Trifemme (at 21B Kamias Rd, Q.C.), they have some designs that are women specific. And their price is usually cheaper than other stores, even cheaper than those in Quiapo and Cartimar.