My period is late and I think I'm pregnant

I had brown spotting for 3 days and then it was gone. And its already a little over a week later and I took a test about 2 days ago and it was bfn?! And I'm planning on taking another pt on Monday (which is my next day off from work) and see then if anything changes. But I will say this I am super gassy, feeling tired, my breast hurt, my lower back hurts, and I feel nauseous sometimes, and I started to break out more in acne (pimples) I have been going to the gym more often to lose weight and get more in shape. I even try getting my husband to go with me whenever he can. But right now I'm feeling so confused and nervous. Has anyone gone through this before?!