stretch marks1 year

I gained maybe around 15 lbs from feb-april and i noticed the appearance of stretch marks on my breasts and hips (a lot on my breasts). Around the beginning of May I noticed my inner thighs had one or 2 littttleee ones, but since have multiplied and grown even though I havent gained any weight since April. (if I have, its at most a lb or 2)

Do stretch marks take time to develop? Could they have happened during that gain and not showing until just now? What are your experiences with them?