Dog suddenly developed separation anxiety and barking during first heat

We have a 7.5 month old Beagle/Stabyhoun mix. She came to us when she was 2 months. I've had several dogs throughout my life, but she is one of the most affectionate and friendly dogs I've ever met.

About two weeks ago, she suddenly started acting very different. She's always been curious and scent driven because of her breed and age, but she began pulling much harder on walks, obsessively sniffing everything, and barking in the evenings. It doesn't seem to matter whether she's physically tired or well rested. She has a solid exercise routine and has no trouble sleeping during the day. We can usually manage the evening barking by tethering her, which helps her settle.

Until she was about 7 months old, she couldn't hold her pee for more than 2 hours. Then, almost overnight, she started being able to hold it for around 5 hours. Up until then, we couldn't leave her alone for more than about 1.5 hours because she needed bathroom breaks. Since my partner and I both work from home, this wasn't really an issue. Whenever we did leave, she would cry briefly and then settle down.

However, about two weeks ago, we realized she was entering her pre heat stage. Since then, everything changed literally overnight. Now, even if just one of us leaves the house while the other stays with her, she cries continuously and can't settle. Tethering or crating doesn't help. The only thing that seems to work is taking her for a short walk for a few minutes to distract her. After we come back inside, she's much calmer. Another thing we've noticed is that, even though she's now able to hold her pee for around 5 hours during the day (until about 7 PM), once the evening barking starts, she suddenly wants to go outside much more frequently.

Yesterday, she officially started her first heat. Has anyone else experienced something like this? Did your dog suddenly struggle with being left alone or start barking more at night during her first heat? Did these behaviors improve after the heat cycle ended?

I'd love to hear your experiences or any advice.