Meron ba kayong company policy regarding tardiness? If meron tignan mo ano ang penalty ng first offense. Ilang beses ka nag-incur ng tardiness? Lagi ba? Or one incident only? If you incurred tardiness on one occassion only, i think suspension as a penalty is stiff. Karaniwan, warning pa lang ang binibigay for first offense.
Were you given a notice to explain why you we're late before you were? Or did you receive a notice of suspension outright without giving you the opportunity to explain your side? If you did not receive a show cause letter then your suspension has procedural defect which will entitle you exemplary damages.
If the infraction as per your facts was already months ago and during that period your attention was not called, there's a presumption of bad faith on the part of the issuing authority. You can always contest the suspension memo. If found that the issuing officer performs in bad faith, you will be entitled to damages. If in the meantime, you already served your suspension and later found your suspension to be illegal, you can demand for the payment of the days you were suspended plus damages.