Is masturbation safe?

Hi All, I'm currently about 22weeks and my husband is not comfortable with having sex with me as he is afraid of being too rough. So we turned to masturbation. I'm ok with that but my problem is after i orgasam, i feel a tight cramp at my tummy for awhile. Which makes me feel like i shouldn't be doing it as it seems to be affecting my tummy. Unsure if it's affecting the baby (i hope not) I'm shy to ask my gyne about this. Is anyone experiencing the same thing?