Vvv faint lines

Ladies I had my son 7 months ago and my husband I are natural family planning. After we’re intimate I test for pregnancy after 12 days. I have been getting very very faint pink lines on the early first response tests since my son has been 2 months. Is there any chance there could be a little hcg left in my system? I’m exclusively breastfeeding and my cycle hasn’t returned. The lines are not getting any darker either. So I really don’t think I’m prego. Should I get blood work done just to make sure? Thanks ladies.