Weird symptoms, could it be copper coil problems?

[Woolyhook]

Hi everyone,
I'm wondering if this would sound familiar to anyone as I'm just not sure what's happening here.
I've had copper coil for 2 years now. All fine really, periods are a bit heavy, take a while to get going but they are regular.
For last 3 weeks or so my tummy has been swollen with a very tight feeling in my uterus. I'm guessing uterus as it feels like a constant braxton hicks contraction. I feel that tightness constantly and really low in my belly.
Doc treated me for a suspected UTI and had urine sent away but urine tested clear at the lab. Doc had originally thought by bladder was inflamed but as urine is clear he thinks it must be my womb. I'm not pregnant, pg test negative.
The one and only time during this swelling that there has been any pain was one time I ran a short distance to nip into a shop. For the next hour or so it felt like I had the worst period pain in history. I fully expected to start bleeding heavily. Then it just went away.
I'm usually fit and healthy so that wee short run wouldn't usually do anything like that!
The fact that doc thinks it's most likely my womb and that there's a coil in there....could it be the coil?
I'm being referred for an ultrasound and my smear test has been brought forward to this week and extra swabs will be done to check for infection.
Just a wee bit worried really.