Please get out of the habit of lying, even about small things. I have known of people for whom it is at best a disordered lifestyle accessory, and at worst a pathology for which help is needed.
I, for one, cannot answer whether lying about how long it’s been since your last confession — especially when you did not try to conceal how long it had been, but tried to make yourself sound “worse” in that respect than you actually were — would rise to the level of mortal sin, in terms of gravity of the matter in itself. It’s not as though you lied about a sin, or concealed a mortal sin. I want to say it isn’t, but neither would I want to mislead you. There also may be a compulsion factor — some people lie so habitually, that they almost can’t help it. Mention all of this to your priest the next time you go to confession, and do what he tells you.
Again, try to break this habit. Also, going to confession once a week is commendable, unless you see it is leading you into scrupulosity. Having not been to confession for “several weeks” is better than most people do nowadays. Better yet, ask your priest how often he thinks you should go.
You cannot go wrong obeying a faithful, doctrinally orthodox priest who teaches and preaches in accord with the magisterium.