When you installed TWRP via fastboot did you use the command "fastboot flash recovery recovery.img" or "fastboot boot recovery.img"? The first of those will install the image "recovery.img" to the recovery partition of the device, overwriting whatever recovery is currently installed. The second will use the image "recovery.img" as a boot image; this will boot you into TWRP just once (allowing you to make a backup or install a ROM) but will not install the recovery image.
My guess is that you did the second. I think it has to be, because I can't think of any other way in which you could now have the stock recovery (it can't reinstall itself, and it doesn't sound like you reinstalled it, in which case the only option left is that you only temporarily booted into TWRP). If this is the case then if you still have the TWRP image on your computer you can use the first command to install it permanently, or the second one to boot into TWRP again without installing it.