What does it mean if the veins in my sack connected to my testicles get bigger? (serious question)

Varicocele: Sounds like you have a varicocele or dilation of veins supplying the testicle. More common on the left side, are generally described as "bag of worms". Varicoceles may cause pain, testis damage/atrophy (generally when they develop in younger males), and infertility. If the varicocele is resulting in one of these symptoms, treatment may be necessary.

Possible varicocele: Varicoceles usually develop on the left side and feel like a "bag of worms" if you feel them when standing. Just make sure there is no lump on your testicle. Check with a urologist if this is bothering you. Varicoceles are by and large harmless but can be associated with male infertility.