Ok, a sudden ramping of use or effort over many days can bring out many different types of issues... And how you're going about it has some bearing.
Doing 5 rides a week, 100 to 150 km, 8-10 hrs & 250-300m ascending is not a heavy schedule, even for starting a season. BUT it CAN BE, depending on what you do on those rides.
Persistent Hard pace, pushing gears, sprints/intervals, overall high effort is not a way to start a riding season after many months off the bike.
Cycling is a repetitive load type use, which can cause repetitive use injuries or body failures... and not being sure about your 'fit', position, posture can be a point of contribution to the issue.
Now that you're into that phase of dealing with an injury/failure, it's best to define the problem more specifically.
There is a difference between 'Pain' and 'Ache'. A well defined injury area is different than one which might be an ache in a specific 'area' of injury.
If its a really specific point, then certainly go see a professional/MD to help define the issue...
If it happens 'after' riding, and only after riding, then it might be a 'Use' issue. If it also happens during or after other activities, then it might be a more general problem.
One point I can offer - Sciatica type condition/injury - also manifests pain in the hamstring muscle/area (back of upper leg) and might indicate some impingment on the sciatic nerve, either spine structural or Piriformis muscle... But certainly worth consulting a Med Professional versed in Sport related injury.
Consider your current Fit, how did you come to that? Any common and widely accepted self-fit methods? If Not, then there are a number of good methods to come to a good start point for bike 'Fit' - from which you can then do adjustments to suit yourself more.
Don;t go Hog-Wild with sudden significant changes to 'try' something or 'fix' something. Small, incremental changes, one at a time and try for while ( a cm change in saddle height is a BIG change...)
Restarting riding, after many months off.
Best to not get into a heavy workout load at the start. Ramp Up slowly over a few weeks/month. Cycling is VERY repetitive and needs to body to re-adapt.
I always focus on easy or moderate pace, with attention to keeping a strong cadence to help rebuild muscle memory in the interaction of muscles (70-90 rpm range) with very occasional harder efforts well after being quite 'warmed' up'. But predominantly steady, moderate for most rides. Slowly ramping up the effort, after the first 20 or so rides...
If I was in your shoes now, with the potential injury/issue and it wasn't getting worse. I would now bring my rides to a steady effort ride, at a pace where I can maintain that 70-90 rpm with the heartrate in Mid-lower Zone 2 or high Z1, for some weeks. With the idea to allow the body to recover. And after apparent 'recovery' I would very slowly ramp up stronger riding again.
OR
You may want to consult with a SPorts/Health Pro now...
Ride On
Yuri