I don't think there's ever a reason to call your midwife just because you're putting on weight, as long as you are otherwise well. You're really small to begin with and you've only put on about 10lbs total in your pregnancy (never mind you lost some, but from your pre-pregnancy weight). That sounds perfectly fine at 26 weeks. Probably because you lost some, your body is trying to make up for that and baby is having a growth spurt. The average weight gain is about 20-26 lbs, so as you lost some, you're likely just putting that back on now.
I started at 15 stone and weigh just a bit over 16 stone at 31 weeks. So I've put on maybe about 16lbs so far. It's come on steadily and slowly, but I've not lost any and I'm much bigger than you anyway. That's considered a healthy weight gain for my weight. It sounds like you're doing fine.
Rapid and sudden weight gain can be a pre eclampsia symptom, but could also just be weight gain/water retention. Mention at your next midwife appt as she'll be checking blood pressure and urine anyway. And keep an eye out for swelling/visual disturbances etc
I honestly wouldn’t worry but maybe book a midwife appointment or just ask her at the next one. I gained LOADS with my first pregnancy because I was small, it’s likely just your body trying to make a sturdier home for your little human