Stretching a Cap That is Too Tight?

I have quite a big head and most ladies' hats won't fit me, especially when I let my hair grow longer. Sometimes I have to buy in the men's section and even those can be too small. I just bought a cotton cap (it looks like one of these old soldier's caps), but it still gives me a headache after some time and I have marks on my forehead. Any idea how I can make it wider?

Thanks in advance.

By Ute White from Brockville, ON