My car started to lose coolant every other week and when I open the expansion tank cap, it builds pressure and leaks from the bottom. The coolant is rusty brownish colour. I've checked the oil and it's fine so don't think oil and coolant are mixing. So far I've been topping it up with water for temp fix. Recently the car stopped blowing hot air, just warm air and it's not warm enough even when car is fully warmed up. The heaters however work on all 4 settings. Now the problem has gotten worse. The car is losing coolant after a short drive, say like 20-30 mins and temperature needle is rising above 90. I drove on motorway at 70mph and temperature needle was fully at 130 and coolant light started flashing. I needed like 1 and half litre of water to top up. I had to pull over to let car cool down for 30 mins then drove it under 50mph with temperature needle between 90 and 3/4. Would anyone know what could be causing this.

Classic symptoms of a blown head gasket. Stop using the car and get it fixed.