I think it is possible that Mildred honestly does not remember taking the overdose of sleeping pills the night before. Encouraged by the society in which she lives, she spends her days living vaguely and vicariously through her artificial "family", the story of whose aimless lives are piped in continuously through the media projected on the three large television screens in her "parlor". Her sole objective in life is the pursuit of immediate, superficial gratification, and so she peruses her soap operas and parties with her friends thoughtlessly day after day. When Montag asks her, "Don't you remember...(what happened) last night", she responds, "Didn't sleep well. Feel terrible...I can't figure it...did we have a wild party or something? Feel like I've a hangover...who was here...hope I didn't do anything foolish at the party".
Mildred's life is filled with "poisons accumulated with the years". She is empty, and in a society which forbids all forms of independent thinking, her mind is anesthetized, she is like a walking zombie. In the rare moments when she does have time to examine her existence, the emptiness of it might drive her to suicide. Or perhaps her mind is so far gone that, as Montag says, "maybe (she) took two pills and forgot and took two more...and (was) so dopey (she) kept right on until (she) had thirty or forty of them in (her)" (Part 1).
Mildred lives in a society where denial of unhappiness is a way of life. They are terminally unhappy, but in denial about it. If they were to admit to their unhappiness, their entire world would crumble. This happens with Montag. When Clarisse asks him, "Are you happy?" it is the catalyst that leads him to admit that no, he isn't. This leads him to asking the question why, finding answers, and trying to solve it. This leads to all of the main action in the book, where he ends up an outcast of society: he loses everything he knew and has to start over. Rather than do this, Mildred, when her unhappiness flares up, overpowering her sense of denial, she tries to escape it all.
Look at how a brief does of unhappiness affected her friend. When Montag reads poetry, Mrs. Phelps "sobbed uncontrollably" and Mrs. Bowles says, "Silly words, silly awful hurting words, why do people want to hurt people" and storms out in a rage. They are so unaccustomed to facing down reality and sorrow, that they deny it exists. When they can't, they try to escape-by storming out, staying busy with life, or like Mildred, taking life. Her denial the next morning is a denial that she is unhappy. Admitting to taking the pills would be admitting she is unhappy, and that is a terrifying truth that she is not equipped to deal with.
Well when Mildred overdose.. she denies that she only took a few.. but when Montag said i filled the bottle up with thirty pills... All Mildred do is watch the "palor walls" all day.. Thats bascially all she knows. When Clarisse met with Montag.. she made him question him self and the life that he lives in... i Think that their society is fulled with secrets..secrets that are kept hidden. If these secrets were to spill out.. the people woudl know that they were living in a BIG lie