Took magnesium citrate (300ml) & now worried because I take a daily magnesium oxide supplement. I feel very nauseous & my stomach hurts after drinking the mag. Citrate. Will this lead to hypermagnesemia? My serum lvl was 2.3 last year. I'm worried.

Unlikely. Your symptoms are likely related to using the mg citrate as a laxative. A1 time occurrence is unlikely to increase your magnesium level. If you continue to feel unwell, get a magnesium level done. All the best!
Probably ok. Skip your daily magnesium today and tomorrow. The nausea and cramping likely due to effect of mag citrate.