The veins at the anal passage plexus have to deal with gravity, so they are prone to statis. Straining of the effort of passing large, bulky stools (such as in a high fibre diet) could be a contributing factor if there was insufficient fluid to soften the stool.
In addition, you mentioned weight training? Using muscles for training or heavy lifting can put additional pressure on the abdomen so may have also had a small part to play. Thats not to say you should give up exercise, just do not do excessive exercises which increase abdominal pressure.
Although people have a tendency to develop these when older, there is no reason why young women shouldn't suffer and indeed they are very common in pregnancy.
Do not be worried about this, a few days will probably see it reduce in size. If there is no pain or bleeding then it is nothing at all to worry about.
Regards
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