dehydration?
sorry, not much else to suggest apart from google!

Dehydration, electrolytes.
I got calf cramps so tight one time that fora couple days it felt like I heard been doing 300# half raises in the weight room.

I'm glad you figured it out, Emma! Hopefully that was it.

We're all different, and I do hope Emma got it right. Here's a paste from my blog:
Everything I'm doing blog

Tuesday, November 27, 2018
Cramps in the swimming pool
Yes, cramps in the swimming pool. I used to get those very reliably about 30 to 40 minutes into a swimming workout. Almost always I got them on my arches. Even if I wasn't pointing my toes I got them. My primary care provider said, well you can try NUUN tablets and tonic water or you can just drink orange juice and see what works for you. I didn't have much success with that. Something else was going on.

I knew that muscle contracture involved a lot of things like sodium pumps, potassium calcium, magnesium and chlorine but after scouring the internet I was still confused about how to address my cramps.
About the same time I was keeping a food diary because I wanted to drop a few pounds that I had picked up over the years. An app called MyFitnessPal tallied how much protein, sodium, fat, calories and other things I was taking in. Those other things included potassium and calcium, but not magnesium. I begin to look at how much sodium, potassium and calcium I was taking in and whether I was getting enough or too much. The app would say zero potassium in some foods that I knew contained potassium. It allowed me to make corrections in the potassium of some foods so I could continue to work with that app. Another handy tool it had--it could scan a barcode on a food product and all I would have to do was specify the amount that I ate.

I consumed enough calcium and sodium but consistently undershot the target for potassium intake (4,700 mg) by a very large amount. After ensuring that the foods I logged did have values for the potassium they contained, I was still short. After a quick search, I amped up my potato consumption. After four or five days of meeting my potassium requirement, I had a swim workout without cramps. Since that time I have not had any significant recurrence. I still try to remember to drink diet tonic water before swimming. I also have some days when I noticeably under shoot the amount of sodium 2,300 mg--those days I feel like I'm about to get cramps, a pretty good signal for me to eat something salty.
Sometimes, I'll drop 2-4 NUUN tablets or other electrolyte product like Emergen-C into a liter water bottle to suck from at poolside.

Because of the complexity of muscle contracture and muscle cramping, I can't guarantee this approach will work for you. If you try this without getting relief then it has to be something else, for which you need a better source of information than myself.

For what it's worth, a very high source of potassium (relative to caloric content) is V8 low sodium vegetable juice (45 Cal, 851 mg per cup. People say, eat a banana to get more potassium. One medium banana (105 cal, 26 gm) only has 422 mg of potassium--good, but not great.

Cheers!

pickle juice, also helps