My savings dropped below nisab value at some point in the past lunar year. So, is zakat still obligatory on me?
The hadiths that mention zakat being one year are all unauthentic.
The actual dues for zakat are given every time you've reached the nisab of that pparticular item. Even if it's 50 times a year. If you've reached the nisab, then you must give the proper portion.
So, if there was a time that your 'zakatable' reached the nisab at some point of time, you should've given zakat. But once the item decreases, then zakat isn't obligatory until you've crossed the limitations.
وَفِي الرِّقَةِ رُبْعُ الْعُشْرِ، فَإِنْ لَمْ تَكُنْ إِلاَّ تِسْعِينَ وَمِائَةً فَلَيْسَ فِيهَا شَىْءٌ، إِلاَّ أَنْ يَشَاءَ رَبُّهَا
Bukhari relates this hadith from Anas, in which Abu bakr had sent a letter saying this is the fardh sadaqah which Allah's Messenger has obligated. At the end of the hadith it says, in the raqqah 1/40 (2.5%), except when there's 199 (dirhams), there's nothing (due) in that, unless its owner wishes.
لَيْسَ فِيمَا دُونَ خَمْسِ أَوَاقٍ مِنَ الْوَرِقِ صَدَقَةٌ وَلَيْسَ فِيمَا دُونَ خَمْسِ ذَوْدٍ مِنَ الإِبِلِ صَدَقَةٌ وَلَيْسَ فِيمَا دُونَ خَمْسَةِ أَوْسُقٍ مِنَ التَّمْرِ صَدَقَةٌ
Muslim relates this hadith from Jabir, in which Allah's Messenger says there is no sadaqah besides (below) 5 awaq wariq. This hadith is also reported from Abu Sa'id without wariq.