Flax like all plants contains estrogenic substances. These compounds are known as phytoestrogens, such as resveratrol, and flavonoids, such as quercetin and rutin. Some plants are higher in these substances than others. Flax is one of the highest sources of phytoestrogens though I have seen being twice as high as soybeans.
Phytoestrogens though are not bad. In fact they help protect us from the dangerous effects of excess estrogens in the body and the even more dangerous xenoestrogens. I have an old post on the subject with more details: