Negative bloods??

Hello ladies! These are my tests (the bottom one is this morning at 13 dpo). The lines are obviously getting darker, however yesterday (12 dpo) I got my blood test done and it was negative? The nurse said it was probably because it’s too early to detect on the blood test (25 mIU is the minimum) and to come back next week when I actually miss my period (due Saturday). Has this happened to anyone else? I’m just terrified because my lower back is killing me and I’ve had medium pms-type cramping that kicked in on Monday after my first positive, no spotting at all though. This is our first month TTC and I feel like we got so lucky I’m just waiting for the other shoe to drop.