Chances in pregnant again?

I'm 3 months pp with my daughter.  I got my period back at 4 weeks. It's been consistent since then and I'm exclusively breastfeeding. My husband and I were intimate last month and we've been using the withdrawal method but 1 time we didn't. I understand that with my body changing my period may come and go since it's only been a few months. I've taken a test already and nothing yet. Any thoughts?