I was told no as well. Its true, you immune system is weakened by the extra work your body is doing producing the breast milk, and if you were to get an infection you can easily pass it to baby before you even knew you had it. I wanted to get one for Erica right after she was born, but couldn't since I was breastfeeding. The way I saw it tho, the tattoo would always be there to get, why risk anything with my precious daughter that I am tattooing my body for?
Also, I got my last one right after I conceived Erica (before I knew I was pregnant) and it hurt like hell. I've got 4 and none had really hurt before that one, but I literally almost passed out. I dunno if it was related tho...