Nacho, indulge me for a brief minute, and I'll share something with you; since, you've already achieved the full lotus - even though for a brief time.
Try this. When you are sitting on the floor with say the left leg in front of you and positioned at a right angle (90 degrees). That is, left thigh straight forward and knee bent so that the lower leg is at a right angle to the thigh and foot pointing towards right. Place your right foot upon your left thigh, as in half-lotus. Now, slowly oscillate the right leg up and down trying to bring the right knee to the ground in a some what modified half-lotus. Eventually, you'll be able to keep the knee on the ground. This helps in developing the stretch for the full lotus. Repeat the same with the other leg.
Eventually, you'll be able to life the other foot (the one at right angle) onto the opposite thigh in full lotus.
Also, when in full lotus breath into it and then stretch (and repeat), and it will help you to maintain it longer. Also, try and massage the entire foot bottom while in full lotus. The sensation is amazing and truly rejuvenating; besides, it distracts you from the otherwise uncomfortable sensations.
But, always use caution, and respect your limitations.
Much Love ....
