Opening lead by partner was ♥7, and the first trick was 7-2-A-Q.
I couldn't understand what the opening lead meant, and I returned another ♥. I didn't return a ♦ because, given our ♦ fit and the extreme bidding by declarer, I was afraid declarer might be void in ♦, as my partner might very likely to have 6 of them by making a 2-level overcall at these colours. Therefore, I played for the declarer to have 5=3=0=5, and for my partner to have 1 ♥ and 6 ♦. It turned out that the declarer was 5=1=1=6, my partner had 5 ♦ only, and my partner led the 7 from ♥743 (I normally played MUD but for this particular partner we didn't have agreement). Declarer ruffed and took 11 tricks eventually. Double dummy analysis said the correct return was either a ♣ or a ♦.
How do I understand the situation of ♥ that I shouldn't return it?