How early can you have systems?

My husband and I have k my been trying for three months and we have been down this road before when we first ttc and then stop. We know it's not as easy as it sounds to get pregnant. So my question is how soon can your body show systems? I've been nauseous for over a week now, I feel very emotional. On top of this my nipples are leaking. Ive had one nipple leak for a couple months and the dr said it was fine they didn't find anything but now both of them are. Is this just stress or me putting it in my head because I want it so bad? I was just in my open window 3 days ago so I know it wasn't from then.