positive First response; negative Clearblue digital

[mslucy]

Hello there.
I had a miscarriage a month ago and felt a bit weird today so I took a pg test.
I tested positive with a first response. Absolutely crystal - no faint lines or anything.
I was completely gobsmacked and asked DH to get me a Clearblue.
Negative.

What is going on?
Could there be some hormones left in my body from the m/c?
I'm pretty sure I ovulated 2 weeks ago - all the signs - and we have "done it" quite a lot.
The plan was to let nature take its way - I'm 40 and the last pg (the mc) was a total shock as I normally don't conceive easily.

Any thoughts would be gratefully received.