Stop worrying!
Firstly, Inheritance Tax (IHT) is completely irrelevant if the value of the estate is less than £325,000. (If that's the case you don't even need to read the rest of this post!).
When calculating the value of the estate, gifts of over £3000 per year, within the last 7 years are taken into account. However any unused portion of the annual £3000 per allowance can be carried over to the following year (but no further). So if you received a gift of £3800 in a year when you'd only received £2200 (or less) in the previous year, you were simply using up your allowance and no part of those gifts will count as part of the estate for IHT purposes.
Even if you did occasionally go over the £3000 annual limit (where you had no transferred allowance from the previous year), it's not going to clobber your mother's estate for a massive amount of tax. If the gift was during the three years prior to your mother's death, the full amount of the excess will count towards the estate. If it was between 3 and 4 years prior to her death, only 80% of the excess will count towards the value of the estate. If it was a year earlier, only 60% counts. In the previous year you only count 40% of the excess. In the year before that just 20% counts.
Note that it's only Inheritance Tax which is relevant here. If your mother had been the richest person in the world she could have given you (through the EPA) billions of pounds a year without a single penny of it counting towards your assessment for Income Tax. (So you've not done anything wrong by not telling the tax man about the gifts you received).
Chris