Probably a combination of your pes planus + new boots + increase in walking.

Definitely rest. Does the pain improve with rest? And get worse with activity?

Keep travelling but reduce/minimise walking until pain is better. Maybe even a day or two. You’ll know its walking/shoe related if reducing activity also reduces pain.

Change your shoes. Likely your new boots are the issue. Recommend something supportive like a good pair of sport shoes. Don’t even try to “wear” in your new boots. They should not be causing this level of pain from day one. Get a new pair and make sure you are fitted properly for both width and length.

Pes planus will also mean you put a lot of pressure on the insides of your feet which will further “jam” your big toe joint. Supportive footwear will help reduce the load on your big toe joint. Orthotic insoles may also help.