According to Organ(y)c online,
Well, most [pads and tampons] are not made of cotton. Many well-known, nationally-advertised brands of tampons, pads and panty liners are made of mostly synthetics. Like rayon. Feminine pads from many of the best known brands also contain something called SAPs, which stands for Super Absorbent Powders. While doing their job to absorb better, SAPs are made from polymers that are derived from crude oil. Think how close those SAPs will be to you.
Tampons and pads often contain wood pulp. It’s not what you expect. But that’s what you’re probably using. Wouldn’t you prefer cotton?