Im 18 weeks with my second baby and i gained 13-14pounds so far(unless it's water weight) i am working out and adding more salt to my food here and there due to low blood pressure. i was 136.6 lbs when i got pregnant. is this to much weight i gained?

No and stop the salt: Lower blood pressure is normal in pregnancy as is your weight gain. And yes, some of the weight is due to fluid retention and amniotic fluid surrounding the baby. The salt will only cause greater fluid retention. Normal blood pressures can be too high in pregnancy and are a sign of pre-eclampsia, a very serious complication requiring hospitalization and sometimes early delivery or C-section.