I only have my period once a year. Does that mean I'm not able to get pregnant except that week?

Needs evaluation. The absence of cycles is amenorrhea. Yes, that makes conceiving very difficult. Lack of menses for that long (when not due to medication) really needs to be evaluated. The lining inside the uterus may need to be sampled to prevent abnormal changes. Also, it is wide to get blood work to check for certain hormonal conditions. I would advise you contact a local gyn md and have an eval. It's wise.