can getting pregnant cause your immune system to go down?

Not sure if I'm pregnant or not yet, I'm only on CD 18 and it's too early to test, but I was wondering if conceiving can cause your body to weaken its immune system. I always get fever blisters on my lip when I'm feeling sick/have the flu and this week I got 6 of them! Other than that, I've been really tired and just sick. Also, is there a supplement I can take besides prenatals to boost my immune system?