Kidney stones but not seen on ultrasound?

[Twosugars]

Hi all, I'm hoping for some advice or experience. Around 2 weeks ago I developed bad pains in my lower abdomen and a fever. My GP diagnosed a UTI and possible kidney stones based on a urine test (blood and leukocytes in urine). Antibiotics helped and sure enough I passed the stone a day or later. Pretty big stone but pain was much better after. Brought it to GP who confirmed it was a stone but the lab couldn't analyse it. I still had some dull pain on and off in the following weeks, GP referred me for renal ultrasound.

I attended the ultrasound yesterday, they said it was all clear, no stones but I've been in excruciating pain since the ultrasound. Couldn't walk or talk, ended up in A&E. I just found out yesterday morning that I'm pregnant which is great news and they ruled out an ectopic via an internal scan. But they can't do a CT scan due to the pregnancy so they sent me home with paracetamol and told me to come back if pain doesn't improve. I was wondering if it's possible that the ultrasound would've missed kidney stones?

The pain is still here and debilitating so I'll be going back to A&E if it doesn't resolve soon. Would appreciate any advice or experience. Has anyone had anything similar? Could it be interstitial cystitis? It's very painful just before and after I pee and there's a slight delay of a few second or 2 before the urine comes out.